antibiotic resistance in the microbial world 13 an...
TRANSCRIPT
Review of Literature
Antibiotic resistance in the microbial world
An historical overview of antibiotics
The antibiotics field was initiated when Paul Ehrlich first
coined the term 'magic bullet', or chemotherapy, to designate
the use of antimicrobial compounds to treat microbial
infections. In 1910, Ehrlich discovered the first antibiotic drug,
Salvarsan, which was used against syphilis. Ehrlich was
followed by Alexander Fleming, who discovered penicillin by
accident in 1928. Then, in the 1935, Gerhard Domagk discovered
the sulfa drugs, thereby paving the way to the discovery of the
anti-TB drug Isoniazid. Then, in 1939, Rene Dubos became the
first scientist to discover an antibiotic after purposely looking
for it in soil microbes. Dubos discovered Gramicidin, which is
still used today to treat skin infections. Finally, in 1943, the first
TB drug, Streptomycin, was discovered by Selman Waksman
and Albert Schatz. Waksman was also the one who coined the
term 'antibiotics'. Thus, antibiotics have been used to treat
bacterial infections since the 1940s [Davey 2000, and Jacoby
1999].
The basic characteristics of antibiotics
Today, there are about 4000 compounds with antibiotic
properties. Antibiotics are used to treat and prevent infections,
and to promote growth in animals.
Antibiotics are derived from three sources: moulds or fungi,
bacteria, or synthetic or semi-synthetic compounds.
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They can be used either internally or topically, and their
function is to either inhibit the growth of pathogens or to kill
them. Antibiotics can thus be divided into Bacteriostatic drugs,
which merely inhibit the growth of the pathogen, and
Bacteriocidal drugs, which actually kill the bacteria. However,
the distinction is not absolute, and depends on the drug
concentra tion, the bacterial species, and the phase of growth.
Antibiotics are more effective against actively growing bacteria,
than against non-growing persisters or spores. When two
antibiotics are used in combination, the effect could be additive,
synergistic, or antagonistic.
Antibiotics can also be divided into broad-spectrum and
narrow-spectrum antibiotics. For example, Tetracycline, a broad
spectrum antibiotic, is active against Gram positive bacteria,
Gram negative bacteria, and even against mycobacteria;
whereas penicillin, which has a relatively narrow spectrum, can
be used mainly against Gram positive bacteria. Other
antibiotics, such as Pyrazinamide, have an even narrower
spectrum, and can be used merely against Mycobacteriulll
tuberculosis.
Modes of action of antibiotics
Antibiotics fight against bacteria by inhibiting certain vital
processes of bacterial cells or metabolism. Based on these
processes, antibiotics can be divided into five major classes
[Brotz-Oesterhelt and Brunner 2008]:
.,. Cell wall inhibitors, such as penicillin and vancomycin
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" Inhibitors of nucleic acid synthesis, such as
fluoroquinolones, which inhibits DNA synthesis, and
rifampin, which inhibits RNA synthesis
" Protein synthesis inhibitors, such as aminoglycoside
" Anti-metabolites, such as the sulfa drugs
" Antibiotics that can damage the membrane of the cell,
such as polymyxin B, gramicidin and daptomycin
eel Wall Synlher.is ~ V8t~"1dO
Bedlracin
"""""'on. c~
Ceph;unyons
Cell Wall Integrity ~.IactlIm_.
Translation::~~""------~~""~""'~~ ProCein~ $ynl/le$i$ ProIein Syntl1esis (50S Inh0tM40t$) (30S In/1ItJi1OfS) E~ T"'~ ~ S<r~ OndamY"" Spocao ... ",on u""""""",, Konamyon
Cytoplasmic Membrane
~ PhOSpholipid Membranes
Figure 2.1 Mode of actions of antibiotics [Pratt 2004]
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The Table 2.1 is a summary of the types or classes of antibiotics
and their properties including their biological source, spectrum
and mode of action [Todar 2004].
Table 2.1 Classes of antibiotics and their properties [Todar 2004]
Chemical class Examples Biological source Spectrum Mode of action (effective against)
Beta-lactams Penicillin G, PellicilliulII Gram-positi \'€, Inhibits steps in (penicillins and Cephalothin /lOrl/rllll! <lnd bcKtcria cell wall cephalosporins) (peptidoglycan)
5vnthesis and
C('/)lll7lo~/)()rilll}IS murein
pecil's assembly
Semisynthetic Ampicillin, GTiJIll-positi\'c Inhibils steps in beta-lactams Amoxicil1in dnd Cram- cell wall
negathT' bacteria (peptidoglycan) s\'nthesis and murein
assembly
Clavulanic Acid Augml'ntin is SlrcplolllY('l'S Cram-positi\'e Inhibitor of
cbvubnic acid cI llVlIi igt' rus and CrJ.rn- bacteritil beta-plus AmoAiciliin negali ve bacteria lactamas~s
Monobactams AzlreClnal11 01 nllll(J/mc feri 111)1 Cram-positive Inhibits steps in I!jolacclIlIl and Cram- cell wall
m'gativ(' bactl'ria (peptidoglycan) synthesis and murein assembly
Carboxypenems Imipt'nl.'m SIrcFiolllYCC5 Cram-positin:~ Inhibits steps in cattlcyll and Gram- cell wall
negative bacteria (peptidoglycan) synthesis and murein assembly
Aminoglycosids Streptomycin 51 I"LptOIll ycc::: gri:::fu::: Cram-positive Inhibits ,mel Cram- translation negative bacteria (protein
synthesis)
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Chemical class Examples Biological source Spectrum lVlode of action (effective against)
17
Gentamicin A1icnmlllllospom Cralll-pasiti \'e Inhibits species and Cram- trans]"tion
negative ba.cteria (protein esp. PsclIdolJlUIlI1S synthesis)
Glycopeptides Vancolll\'cin Alllycoil1topsi5 Cram-positive Inhibits steps in orit'lll{/ii~No(a}'di(/ badl'rid, esp. mUTl'in oricil tlllis(formcrly SlnpllylococC//s (peptidoglycan) designated) IlIlYL'/IS biosynthesis
and assembly
Lincomyci ns Clindamycin Sln'll/oIllYccs Cram-positive Inhibits !illeo/llcllsi" and Cralll- translation
negative bacteria (protein esp. syntheSis) afklC'fO hie Bactcrois
Macrolides Erythroll1:"cin, Stl'CptOlllYCCS Cram-positive Inhibit Azithromycin cryfhreus bcKteria, Gram- translation
neg,ltive bacteria (protein not syntheSis) enterics, Neisseria, LcSiolld/l1, MycoplllSlll1l
Polypeptides Polymyxin Bilcilllls polylllyxa Gram-negative Damages bactt;'ria cytoplasmic
membranes
Bacitracin Bilcill1l5 slIl1tilis Cram-positive Inhibits steps in bactt.'rid murein
(peptidoglycan) biosynthesis and assemblv
Polyenes Amphotericin Slrcplolllyt"{'s fungi Indctivatc 1l0dOS/IS (l-I i S tOpiIlSIIlI1) membranes
containing stE'rols
Nystatin Strcpto/J/YCf!S Fungi (Calldido) Indctivate /lOurSCI membranes
containing
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Chemical class Examples Biological source Spectrum Mode of action (effective 18 against)
stewls
Rifarnycins Rifampicin SlrcI'iOlIlYCCo;; Cr,llll-positive Inhibits IIIcriitcrn1l1ci and Cram- transcription
negative bacteria, (bacteri"i RNA MycohilCfcriulI1 polymerase) tllberculosis
Tetracyclines Tetracycline Sf fc}/II 1I11yCC5S pecies Gram-positi \'12 Inhibit and Crcllll- tran~btiol1
negiltive bacteria, (protein Rickettsias synthesis)
Semisynthetic Doxycycline Gram-positivE' Inhibit tetracycline Jnd Cram- translation
negative bacteria, (proh.'in Rickettsias synthesis) Ehrlichia, Borrelia
Chloramphenicol Chloramphenicol Slrcp/oIIIYCL'5 GrJn1-positi\'e Inhibits z'e/fez IIC/Ilt' and Gram- tral151,ltiol1
negali ve bacteria (protein synthesis)
Quinolones Nalidixic acid synthetic ~lainly Cram- Inhibits DNA llegJtive bacteria replication
Fluoroquinolones Ciprofloxacin synthetic Gram-negali\"e Inhibits DNA and some Cram- replication positive bacteria (Bacillus tlllfhmo's)
Growth factor Sulfanilamide, svnthetic Cralll-positi\-e Inhibits folic analogs Cantrisin, and Gram- ,lCid
Trimethoprim negative bacteria metabolism (anti-fobte)
Isoniazid (I\JH) synthetic A1yco/Jl1cferilllll Inhibits tll/Jerculosis mycolic acid
synthesis; analog of pyridoxine (Vit
Chemical class Examples
para-Jlllinosa licylic acid (PAS)
Biological source
synthetic
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Spectrum (effective against)
Myco/)!lctcrilllli fu/)crclIlosis
Mode of action
86)
Anti-fol'lte
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Complexity of Antimicrobial Therapy
Drug Retention Problems
Eliminati 0 n from Host, Inactivation by Host
WrongDrug .... Wrong Spectrum ,..
of Activity (illfonred be,! guo,,),
Few Targets on Patha gen (fungi, protozoa, viruses)
~ Drug Delivery .., Problems Oral (destruction or poor uptake),
Inttavenous or Intramuscular (inconvenient), Topical,
Poor Ti ssue Uptake, Inj ection into Body Cavity
__ Side Effects Toxicity to Host,
Allergic Reaction, Normal Flora Disruption
( superinfection)
Selective Toxicity and Successnd Delivery
Development of .... Resistance ,..
Evasion, Mutation-Mediated,
TOXUlS
Exotoxins, Endotoxins
Acquired(R plasmids) • (in Gram-negative septicemia antibiotic treatment can even
.. make situation worse) Prevent Resistanc e b y Using: 1r.""""''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''Il
Only as Necessary, At sufficiently high doses
for suffi ci entl y Ion g p eri 0 d s, In Combination
Prevention of Growth Bactericidal,
B a cteri 0 stati c, Host Defenses
(elimination of pathogen from body)
Figure 2.2 Complexity of antimicrobial therapy [Lorian and Victor 2005]
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Drug resistance
The scope of the drug resistance problem
Bacteria clearly have a wondrous array of biochemical and
genetic systems for ensuring the evolution and dissemination of
antibiotic resistance [Medeiros 1997].
Drug resistant bacteria have been posing a major challenge to
the effective control of bacterial infections for quite some time.
Drug resistance refers to a situation in which the drugs that
usually destroy the bacteria no longer do so. It implies that
people can no longer be effectively treated against the bacteria.
Consequently, they are ill for longer periods of time; and they
face a greater risk of dying. Furthermore, epidemics are
prolonged, putting more people at a risk of becoming infected.
Antibiotic resistance is an extremely expensive problem. Its
costs in the US alone are estimated at US $5-$24 billion per year
[McGowan, Jr 2001].
Causes for drug resistance
,., One of the main causes of antibiotics drug resistance IS
antibiotic overuse, abuse, and in some cases, misuse, due
to incorrect diagnosis [Okeke et. AI. 1999].
,., A second cause is counterfeit drugs. Antibiotic use 111
animal husbandry is also creating some drug resistant
bacteria, which can be transmitted to humans [WHO 2002,
Feinman 1998].
,., Increased globalisation could also cause the spread of
drug resistance [MacPherson et. al. 2009].
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'r Finally, hospital settings often gIve rise to antibiotic
resistant bacteria [Weinstein 2001].
Natural (intrinsic) and acquired resistance
Antibiotic resistance can be divided into natural resistance and
acquired resistance. Natural resistance means that the bacteria
are 'intrinsically' resistant. For example, Streptomyces has some
genes responsible for resistance to its own antibiotic or
vancomycin resistance 111 Escherichia coli. Other examples
include organisms that lack a transport system or a target for
the antibiotics. In other cases, the resistance can be due to
increased efflux activity.
Acquired resistance refers to bacteria that are usually sensitive
to antibiotics, but are liable to develop resistance. Acquired
resistance is due to mutations in chromosomal genes, or by the
acquisition of mobile genetic elements, such as plasmids
[Gascoyne-Binzi 1994] or transposons [Grubb 1998], which carry
the antibiotic resistance genes
Genetic and phenotypic resistance
Broadly speaking, antibiotic resistance could also be divided
into genetic drug resistance, which is the one most commonly
discussed, and phenotypic drug resistance, which is a more
subtle type [Streiche et. al. 2004]. Genetic resistance is due to
chromosomal mutations or acquisition of antibiotic resistance
genes on plasm ids or transposons. Phenotypic resistance is due
to changes in the bacterial physiological state, such as the
stationary phase, antibiotic persisters [Balaban et. al. 2004], and
the dormant state.
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Genetic drug resistance mechanisms
Until the 1950s, it was not clear how the bacteria acquire drug
resistance. Then, Joshua Lederberg devised replica plating, and
demonstrated that the antibiotic resistant mutants are pre
existing [Hughes and Datta 1983]. Thus, the antibiotics merely
selected these mutants. Then, in 1988, John Cairns showed that
when the bacteria are not growing, they are nevertheless able to
acquire new mutations, due to some genetic alteration process.
The introduction of streptomycin for treating tuberculosis was
thwarted by the rapid development of resistance by mutation of
the target genes. Mutation is now recognized as the commonest
mechanism of resistance development 111 Mycobacterium
tuberculosis, and the molecular nature of the mutations
conferring resistance to most antituberculosis drugs is now
known [Musser 1995]. Those mutations are called adaptive
mutations. It was never formally proven that adaptive
mutations cause antibiotic resistance; however, it is possible,
particularly in non-growing forms of bacteria.
As the evolutionary time frame has to be less than 50 years it is
not possible to derive a model in which evolution could have
occurred by mutation alone from common ancestral genes. They
must have been derived from a large and diverse gene pool
presumably already occurring in environmental bacteria. Many
bacteria and fungi that prod uce antibiotics possess resistance
determinants that are similar to those found in clinical bacteria.
[Davies 1997]. Gene exchange might occur in soil or, more
likely, in the gut of humans or animals [Davies 1997]. It has
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been discovered that commercial antibiotic preparations contain
DNA from the producing organism, and antibiotic resistance
gene sequences can be identified by the polymerase chain
reaction. [Webb and Davies 1993].
There are five major mechanisms of antibiotic drug resistance,
which are due to chromosomal mutations:
,. Reduced permeability or uptake.
,.. Enhanced efflux.
,. Enzymatic inactivation.
',. Alteration or over-expression of the drug target.
,. Loss of enzymes involved in drug activation.
Efflux purrp
Antibiotic
Antibiotic
Fig 2.3 Four major biochemical mechanisms of antibiotic
resistance [To dar 2008J
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Multi-drug resistance (MDR) resistance
The multi-drug resistance mechanism can be caused by
different mechanisms in different organisms. For example, in
1959, the Japanese found Shigella species that were resistant to
Su Ifonamides, Streptomycin, Chloramphenicol, & Tetracycline.
The resistancewas due to plasmid, which carried different
antibiotic resistance genes [Hughes and Datta (1983)]. The other
MDR mechanism IS due to sequential accumulation of
chromosomal mutations in different drug resistant genes, as 111
the case of MDR- TB and XDR-TB
Examples of chromosomal mutations
Let us now examine some examples of chromosomal mutations.
A. Decreased uptake
Antibiotic modification is the best known: the resistant bacteria
retain the same sensitive target as antibiotic sensitive strains,
but the antibiotic is prevented from reaching it. This happens,
for example, with ~ lactamases-the ~ lactamase enzymatically
cleaves the four membered ~ lactam ring, rendering the
antibiotic inactive. Over 200 types of ~ lactamase have been
described. Most ~ lactamases act to some degree against both
penicillins and cephalosporins; others are more
specific-namely, cephalosporinases
enzyme found in Ellterobacter spp)
(for example, AmpC
or penicillinases (for
example, Staphylococcus allreus penicillinase). ~ Lactamases are
widespread among many bacterial species (both Gram positive
and Gram negative) and exhibit varY1l1g degrees of inhibition
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by P lactamase inhibitors, such as clavulanic acid [Livermore
1995].
Neisseria gonorrhoea porin can acquire mutations that can cause
resistance to penicillin and tetracycline [Davies 1997]. Another
example is Enicro/lacier aerogcllcs porin, which can acqUlre
mutations that cause cephalosporin resistance.
B. Increased efflux activity
Some antibiotic resistant bacteria protect the target of antibiotic
action by preventing the antibiotic from entering the cell or
pumping it out faster than it can flow in (rather like a bilge
pump in a boat). P Lactam antibiotics in Gram negative bacteria
gain access to the cell that depends on the antibiotic, through a
water filled hollow membrane protein known as a porin. In the
case of imipenem resistant Pseudolllollas aerugillosa, lack of the
specific D2 porin confers resistance, as imipenem cannot
penetrate the cell. This mechanism is also seen with low level
resistance to fluoroquinolones and aminoglycosides. Increased
efflux via an energy-requiring transport pump is a well
recognized mechanism for resistance to tetracyclines and is
encoded by a wide range of related genes, such as tet(A), that
have become distribu ted in the enterobacteriaceae. [Chopra,
Hawkey et. al. 1992].
Tetracycline efflux was discovered in the early 1980s. TetK
serves as an example for an efflux-mediated Tetracycline
resistance [Chopra, Hawkey et. al. 1992]. Under normal
conditions, the efflux gene, TetK, is not expressed, due to a
suppressor that is bound to the promoter region. However, in
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the presence of Tetracycline, it binds to the repressor, relieves
the suppression, and causes transcription and translation of the
efflux pump, thereby leading to Tetracycline resistance.
C. Enzymatic inactivation
Antibiotic modification is the best known: the resistant bacteria
retain the same sensitive target as antibiotic sensitive strains,
but the antibiotic is prevented from reaching it. This happens,
for example, with ~ lactamases-the ~ lactamase enzymatically
cleaves the four membered ~lactam nng, rendering the
antibiotic inactive. Over 200 types of ~ lactamase have been
described. Most ~ lactamases act to some degree against both
penicillins and cephalosporins; others are more
specific-namely, cephalosporinases (for example, AmpC
enzyme found lt1 Elllero/Jaclcr spp. or penicillinases (for
example, Staphylococclis aureus penicillinase). ~ Lactamases are
widespread among many bacterial species (both Gram positive
and Gram negative) and exhibit varying degrees of inhibition
by ~ lactamase inhibitors, such as clavulanic acid [Livermore
1995].
D. Alteration of drug target / production of an alternative Target
Another mechanism by which bacteria may protect themselves
from antibiotics is the production of an alternative target
(usually an enzyme) that is resistant to inhibition by the
antibiotic while continuing to produce the original sensitive
target. This allows bacteria to survive in the face of selection:
the alternative enzyme "bypasses" the effect of the antibiotic.
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The best known example of this mechanism is probably the
alternative penicillin binding protein (PBP2a), which IS
produced in addition to the "normal" penicillin binding proteins
by methicillin resistant Stllphylococcus IllireliS (MRSA). The
protein is encoded by the mecA gene, and because PBP2a is not
inhibited by antibiotics such as flucloxacillin the cell continues
to synthesize peptidoglycan and hence has a structurally sound
cell wall. [Michel and Gutmann 1997]. The appearance III
1987 of vancomyclll resistant enterococci has aroused much
interest because the genes involved can be transferred to S
aureus, and this can thus theoretically result in a vancomycin
resistant MRSA. The mechanism also represents a variant of the
alternative target mechanism of resistance [Leclercq and
Courvalin 1997]. In enterococci sensitive to vancomycin the
normal target of vancomycin is a cell wall precursor that
contains a penta peptide that has a D-alanine-D-alanine
terminus, to which the vancomycin binds, preventing further
cell wall synthesis. If an enterococcus acquires the vanA gene
cluster, however, it can now make an alternative cell wall
precursor ending in D-alanine-D-Iactate, to which vancomycin
does not bind.
Most strains of Streptococcus PllCll1llOlllilC are highly susceptible
to both penicillins and cephalosporins but can acquire DNA
from other bacteria, which changes the enzyme so that they
develop a low affinity for penicillins and hence become
resistant to inhibition by penicillins [Tomasz and Munoz 1995].
The altered enzyme still synthesize peptidoglycan but it now
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has a different structure [Garcia-Bustos and Tomasz 1990].
Mutants of Streptococclis pyogencs that are resistant to penicillin
and express altered penicillin binding proteins can be selected
in the laboratory, but they have not been seen in patients,
possibly because the cell wall can no longer bind the anti
phagocytic M protein.
E. Loss of enzymes in drug activation
The loss of enzymes involved in drug activation is a relativelv
new mechanism of drug resistance. In this case, the antibiotic
itself is a prodrug, which has no direct activity against the
bacteria. Rather, it relies on the activation by a bacterial
enzyme. INH can serve as a useful example. KatG (catalase
peroxidase) [Heym et. al. 1992] is an enzyme involved in the
activation of INH, which produces a range of reactive
metabolites including reactive oxygen species and then reactive
organic radicals, which then inhibit multiple targets, including
mycolic acid synthesis [Yu et. al. 2003]. Another example is the
metronidazole (MTZ) prod rug. MTZ is activated through RdxA
(nitroreductase), and then forms reactive species that damage
the DNA. Thus, mutations in this enzyme cause resistance to
Metronidazole [Sisson et. al. 2002].
Regulation of resistance genes
Bacteria are extremely versatile 111 becoming resistant to
antibiotics, and are actually able to regulate their drug
resistance genes. One example is due to repressors, as in the
case of tetracycline, efflux mediated drug resistance (discussed
earlier) [Chopra and Hawkey et. al. 1992]. A second example,
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which relates to erythromycin resistance genes (erm) is due to
attenuation [Bozdogan et. a!. 2004]. In the absence of
erythromycin, a stem-loop structure forms in the mRNA, which
buries the Ribosome Binding Site (RBS) and the start codon.
Thus, in the absence of the antibiotics, the drug resistance gene
is not expressed. However, low concentrations of erythromycin
cause the RBS and start codon to be exposed, causing a
translation of the drug resistance gene, erm, resulting in the
expression the gene.
Transfer of resistance genes
In addition to chromosomal mutations, a second broad category
of drug resistance is due to mobile genetic elements, such as
plasmids or transposons, which carry drug resistant genes. Few
examples are:
Streptomycin-resistance genes, strA- and strB, which can be
carried on plasmid, and cause Streptomycin resistant [Hughes
and Datta 1983].
Sulfa drug resistance, caused by plasmids that carry the drug
insensitive form of the enzyme [Wise and Abou-Donia (1975)].
A relatively new mechanism is the plasmid-mediated qnr
(quinolone resistance). The qllr gene encodes a device called
pentapeptide, which is a DNA mimic. Penta peptide binds to the
DNA gyrase and thus helps prevent the quinolone drug from
binding to the gyrase, thereby ca Llsing low-level resistance
[Tran and Jacoby 2002].
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Transposons can also carry drug resistant genes [Gascoyne
Binzi 1994]. It is noteworthy that plasmids and transposons are
not involved in drug resistance mechanisms in TB.
Phenotypic drug resistance
Phenotypic drug resistance refers to the fact that when the
bacteria are not growing, they can become unsusceptible to
antibiotics [Bronstad 1996]. Then, when the bacteria are sub
cultured into a fresh media, and they begin to grow again, they
regain their antibiotic susceptibility [Streiche, et. al. 2004]. This
complex mechanism has been posing significant problems as in
biofilm infections and particularly for TB chemotherapy.
BiofiIm infections
Bacteria that adhere to implanted medical devices or damaged
tissue can encase themselves 111 a hydrated matrix of
polysaccharide and protein, and form a slimy layer known as a
biofilm.
Biofilms have been found to be involved in a wide variety of
microbial infections in the body, by one estimate 80% of all
infections [NIH 2002]. Infectious processes in which biofilms
have been implicated include common problems such
as unnary
infections,
tract infections, ca theter infections, middle-ear
formation of dental plaque, gingivitis,
coating contact lenses [Rogers 2008], and less common hut more
lethal processes such as endocarditis, infections in cystic
fibrosis, and infections of permanent indwelling devices such as
joint prostheses and heart valves [Lewis 2001, Parsek and Singh
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2003]. More recently, it has been noted that bacterial biofilms
may impair cutaneous wound healing and reduce topical
antibacterial efficiency 111 healing or treating infected skin
wounds [Davis et. al. 2008].
Microbial biofilms not only serve as a nidus for disease but also
are often associated with high-level antimicrobial resistance, a
consistent phenomenon that may explain the persistence of
many infections in the face of appropriate antimicrobial therapy
[Donlan 2002, Schachter 2003].
The mechanisms of resistance in biofilms are different from the
now familiar plasmids, transposons, and mutations that confer
innate resistance to individual bacterial cells. In biofilms,
resistance seems to depend on multicellular strategies [Stewart
and Costerton 2001].
Characterizing the antibiotic resistance of biofilms as "tenacious
survival rather than aggressive virulence", Stewart and
Costerton made four hypotheses:
, the drug fails to penetrate beyond the biofilm surface
layer;
, some bacteria differentiate into a protective phenotypic
state;
, antibiotic action IS antagonized within the regions of
nutrient depletion or waste production.
,. fourth mechanism, referred to as persistence. Persistence
has been seen, for example, in PseudollloIJas aerllgiIJosa,
where increased expression of the regulatory gene PvrR,
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which keeps the bacteria in biofilms, renders them
insensitive to a wide range of different antibiotics
For example, some orthopaedic devices can have StaphylococcliS
al/reliS and Staphylococcus epidermidis infections. Once these
devices are infected with the biofilm, it is extremely difficult to
eliminate the biofilm completely merely by using antibiotics.
Often, the orthopaedic device must be replaced [Stewart
and Cos terton 2001].
Biofilm formation
Initially, the bacteria simply attaches to surfaces irreversibly,
and then irreversibly. Thus, early biofilms are formed, and turn
into mature biofilms. Such biofilms are able to release new
organisms off the structure. Biofilm bacteria are extremely
resistant to antibiotics. When the susceptibility of the
planktonic form and biofilm, is compared, it is observed that
antibiotic imipenem can destroy planktonic organisms of
Pseudomollas aerugillosa effectively at 1 (pg/ml), but require at
least 1024 (pg/ml) to fight against biofilm [Cornel is 2008].
The Biofilm structure is extremely complex. The bacteria are
divided into different SUb-populations, ranging from an almost
spore-like sub-population, to a more actively metabolizing
population at the colony surface.
Salicylate-induced antibiotic resistance
Another form of phenotypic drug resistance IS mediated by
salicylic acid, which is the active component in aspirin.
Different organisms have been found to have salicylate-
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mediated drug resistance. Escherichia coli is the best example,
and additional ones include Klebsiella, Pseudomonas,
BurkllOlderia, and also MycolJt7cteriulII tuberculosis. It has been
demonstrated that in the presence of salicylate, TB bacteria is
less susceptible to INH, Rifampicin, EMB, and PAS. Preliminary
experiments in the mouse model of TB demonstrate that aspirin
can antagonize the activity of INH, indicating that it might also
have some effect in-vivo.
The mechanism of Salicylate-induced antibiotic resistance III
Escherichia coli is as follows:
Multiple Antibiotics Resistance (MAR) operon in the Escherichia
coli where the MarR is the repressor. Salicylate binds to MarR in
order to release the suppression of the MarAB operon. MarA
encodes a transcription factor, which in turn, activates the
transcription of the efflux pump acrAB, as well as the
membrane channel tolC, which is required for the functioning
of the pump. Thus, the first drug resistance mechanism IS
conducted through increased efflux [Cohen et. al. 1993].
In a second mechanism, MarA enhances the transcription of
micF, an antisense RNA for ompF, a membrane porin required
for entry of antibiotics. Thus, micF shuts down the expression
of ompF through antisense. When the porin expression is
reduced, the drug intake is reduced as well [Price et. al. 2000].
Bacterial persisters
The bacterial persisters are an important example of the
phenotypic resistance. Persistence was first discovered with
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penicillin in 1944. Joseph Bigger demonstrated that penicillin
can kill merely 99% of the bacteria. The remaining 1 % of the
bacteria were persisters. When these persisters were cui tu red to
fresh media, they regained susceptibility to antibiotics [Balaban
et. al. 2004, Klingenberg 2007].
Toxin-Antitoxin (T A) model
For many years, the mechanism of persisted resistance to
antibiotics remained unknown. Then, in the 1980s, Harris
Moyed found the HipA gene being involved in persistence in
E.coli. Later, a group headed by Kim Lewis discovered that
HipA and HipB form a toxin antitoxin (TA) module, in which
an inappropriate expressIOn of toxin leads to persister
formation. The TA model was initially discovered on plasmids,
but was later observed in chromosomes of Illany bacterial
species. When a toxin is expressed, it shuts down transcription
and translation. Thus, the activity of toxins and anti-toxins
must be carefully regulated, to prevent cells from dying
[Schumacher et. al. 2009].
The model has difficulties explaining persistence in orgal1Isms
that do not have TA modules. More recently, a Chicago group
has demonstrated that if any toxic proteins are expressed, they
can induce persister formation, regardless of the toxin-antitoxin
module. These findings raise some questions as to the validity
of the toxin-antitoxin theory.
35
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PhoU
Recently, we have identified a new persisted gene, PhoU,
through a transposon-based screen in Escherichia coli. The PhoU
mutant displays high susceptibility to a range of different
antibiotics, such as ampicillin, streptomycin, sulfa drugs, and
quinolone drugs [Li and Zhang 2007]. It is also more susceptible
to different conditions, such as heat, starvation, acid pH, and
weak acids. A wild type PhoU gene can complement PhoU
mutant phenotypes.
An interesting feature of the PhoU mutant is that it is highly
susceptible to ampicillin in the stationary phase. Many other
antibiotics, especially penicillin, are not active against
stationary phase bacteria, but merely against growing bacteria.
We have also demonstrated, through microarray experiments,
that the PhoU mutant has a hyperactive metabolism.
Thus, PhoU appears to be a suppressor mechanism for cellular
metabolism. When it is expressed, it shuts down cellular
metabolism. Although the detailed mechanism is not clear yet,
it is believed that PhoU could be an interesting drug target for
killing persister bacteria [Smith and Romesberg 2007].
Managing the drug resistance problem
Limiting the Spread of Drug Resistant Bacteria
Several measures could be used to prevent the spread of drug
resistant bacteria [O'Fallon et. al. 2009]:
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Jy the use of better treatment strategies; better immunization
programmes; improved hygiene and nutrition; and
initiatives targeting the poor populations.
Jy it might be useful to establish antibiotic resistance
surveillance programmes.
,. better education of health care professionals is required to
prevent the prescription of unnecessary antibiotics.
,. It is noteworthy that significant investment of time, effort,
and money is necessary in order to control antibiotic
resistant bacteria. Of course, as long as antibiotics are
used, antibiotics resistance is bound to occur. However, it
IS possible to reduce the drug resistance problem
[O'Fallon et. a1. 2009]:
Jy to ensure that antibiotics are used only when necessary.
,.. to ensure that they are used for the appropriate amount of
time; that is, that the treatment is not stopped before it is
completed. Patient compliance is a key problem in that
respect.
,.. strategy for limiting drug resistance is to use antibiotics
combinations.
Unfortunately, while all these strategies seem sound 111 theory,
in reality, the problem persists.
Development of new antibiotics
Another possibility is to develop new antibiotics. However, that
is not an easy task. The sad irony is that many pharmaceutical
37
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compal1les have decided to abandon their antibiotic
development programmes when new antibiotics are needed
most, since 99% of the drug candidates fail, and antibiotics are
not as profitable as other, more commonly used, drugs. The
traditional approach of screening microbes for antibiotics is not
efficient [Silver and Bostian 1993].
A second approach, which utilizes target-based screel1lng,
became popular when genomlcs tools became available.
However, although the idea is appealing, in reality, it is
extremely difficult. Many companies have tried this approach,
and so far they have all failed. The whole organism-based
approach is more feasible but the conditions of screen need
careful consideration [The Lancet 2009].
Mobilization of host defense mechanisms
Yet another approach is to mobilize host defense mechanisms.
This can be achieved through the mobilization of innate
immunity such as defensins, or through the development of
vaccines, which make antibiotics less necessary. The idea is to
boast the immune response capability to control the bacterial
infection. Of course, that approach is not always successful.
The use of normal bacterial flora
Finally, one could also potentially use normal bacterial flora to
suppress some pathogens.
Conclusion
To quote the Nobel Prize laureate Joshua Lederberg: "Antibiotic
resistance as a phenomenon is, in itself, not surprising. Nor is it
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new. It is, however, newly worrying, because it is accumulating
and accelerating, while the world's tools for combating it
decrease in power and number."
This description may sound gloomy, but unfortunately, it is
rather precise. One must remember that the bugs have been on
this planet much longer than the human race and can develop
resistance to any antibiotics used to treat them. Hence, one has
to use a combination of approaches as discussed above to
mll1lmlZe the resistance problem, and hopefully can live 111
peace with the microbes.
39
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Nosocomial (leU) Infection
CDC and NNIS system
The Centers for Disease Control and Prevention (CDC's)
National Nosocomial Infections Surveillance (NNIS) system has
been serving as an aggregating institution for 30 years. The
NNIS system is a voluntary, hospital-based reporting system
established to monitor hospital-acquired infections and guide
the prevention efforts of infection control practitioners. Patients
in intensive-care units (ICUs) are at high risk for nosocomial
infections and since 1987 have been monitored in the NNIS
system by site-specific, risk-adjusted infection rates according
to ICU type [Garner 1996].
Definitions of nosocomial infection
The NNIS System defines a nosocomial infection as a localized
or systemic condition
,.. that results from an adverse reaction to the presence of an
infectious agent(s) or to its toxin(s) and
,.. that was not present or incubating at the time of
admission to the hospital For the most nosocomial
infections, this means that the infection becomes usually
evident 48 hours (i.e., the typical incubation period) or
more after after admission.
However, because the incubation period varies with the type of
pathogen and to some extent with the patient's underlying
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conditions, each infections must be assessed individually for
evidence that links it to the hospitalization [Garner 1996].
According to another definition, it states that nosocomial
infections are those infections that are the result of treatment in
a hospital or a healthcare service unit. Infections are considered
nosocomial if they first appear 48 hours or more after hospital
admission or within 30 days after discharge This type of
infection is also known as a hospital-acquired infection (or, in
generic terms, healthcare-associated infection) [Eggimann &
Pi ttet 2001].
In the United States, the Centers for Disease Control and
Prevention estimates that roughly 1.7 million hospital
associated infections, from all types of bacteria combined, cause
or contribute to 99,000 deaths each year [Pollack 2010].
The connection between the high death rate of hospitalized
patients and the exposure of patients to infectious
microorganisms was first made in the mid-nineteenth century.
Hungarian physician Ignaz Semmelweis (1818-1865) noted the
high rate of death from puerperal fever in women who
delivered babies at the Vienna General Hospital. At about the
same time, the British surgeon Joseph Lister (1827-1912) also
recognized the importance of hygienic conditions in the
operating theatre. His use of phenolic solutions as sprays over
surgical wounds helped lessen the spread of microorganisms
resident in the hospital to the patient. He recognized that
infections could be transferred from the surgeon to the patient.
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Lister's actions spurred a series of steps over the next century,
which has culminated in today's observance of sterile or near
sterile conditions in the operating theatre [Ruef 2005].
Nosocomial infections occur worldwide, both in the developed
and developing world. They are a significant burden to
patients and public health. They are a major cause of death and
increased morbidity in hospitalized patients. They may cause
increased functional disability and emotional stress and may
lead to conditions that reduce quality of life. Not only do they
affect the general health of patients, but they are also a huge
burden financially. The greatest contributors to these costs are
the increased stays tha t pa tien ts with nosocomial infections
require [Vincent et. al. 1995, Ruef 2005].
Nosocomial infections are most frequently infections of the
urinary tract, surgical wounds, and the lower respiratory tract.
A World Health Organization prevalence study and other
studies have shown that these infections most commonly occur
in intensive care units (ICUs) and in acute surgical and
orthopedic wards [Ruef 2005]. Infection rates are also higher in
patients with increased susceptibility due to old age,
underlying disease, or chemotherapy. They are susceptible to
infection because of their underlying diseases or conditions
associated with impaired immunity as well as several violations
of their immune system or risks of aseptic mistakes in patient
management during invasive monitoring and they are prone to
secondary infections after exposure to broad -spectrum
antimicrobials [Eggimann and Pittet 2001].
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Factors that influence infection
Nosocomial infections, may be exogenous or endogenous in
origin. The exogenous source may be another person in hospital
(cross infection) or a contaminated item of equipment or
building service (environmental infection). A high proportion
of clinically apparent hospital infections endogenous (self
infection), the infecting organisms being derived from the
patient's own skin, gastro-intestinal or upper respiratory flora
[Ruef 2005, Benn 1985].
Most infections acquired III hospital are caused by
microorganisms that are commonly present as commensal in the
general population. Thus, contact with microorganisms is
seldom the sole or main event predisposing to infection, various
risk factors, alone or in combination, influence the frequency
and nature of hospital infection.
Susceptibility to infection
The National Nosocomial Infection Surveillance System
database compiled by the CDC shows that the risk factors that
increase the opportunity for hospitalized individuals to acquire
infections are:
'Y a prolonged hospital stay
, severity of underlying illness
,. compromised nutritional or immune status
'Y use of indwelling catheters
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,.. failure of health care workers to wash their hands
between patients or before procedures
,.. prevalence of antibiotic-resistant bacteria from the
overuse of antibiotics
Natural resistance to infection IS lower 111 infants and the
elderly, who often constitute the majority of hospital patients.
Preexisting disease, such as diabetes, or other conditions for
which the patient was admitted to hospital, and the medical or
surgical treatment, including immunosuppressive drugs,
radiotherapy or splenectomy, may also reduce the patient's
natural resistance to disease. Moreover, the natural defense
mechanisms of the body surfaces may be bypassed either by
injury or by procedures such as surgery, insertion of an
indwelling catheter, tracheotomy or ventilatory support
[Wenzel 1983].
Contact with other patients and staff
In common with large institution or workplace, the patients and
staff of a hospital share many facilities in close or crowded
conditions. Admitting infected patients or carriers for treatment
clearly serves as a potential source of infection of others.
Patients with comparable susceptibility to infection tend to be
concentrated in the same area, e.g. in neonatal units, burns
units or urological wards, where infected and non-infected
patients may be cared for by the same staff, thus creating
nu merous opportunities for the spread of microorganisms by
direct contact. The more susceptible patients usually require the
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most intensive care with far more daily contacts with staff who
act as vectors in the transmission of microbes like insects
spreading parasites [Wenzel 2003].
Inanimate reservoirs of infection
Equipment and materials in use in hospitals often become
contaminated with microorganisms, which may subsequently
be transferred to susceptible body sites on patients.
Gram-positive cocci, derived from the body flora of the hospital
population, are found in the air, dust, and on surfaces where
they 111ay survive along with fungal and bacterial spores of
environmental origin.
Gram-negative aerobic bacilli are common in moist situations
and in fluids, where they often survive for long periods, and
may even multiply 111 the presence of minimal nutrients. An
important example of this is legionelle in hospital domestic
wa ter supplies.
Awareness of the common reservOirs of environmental and
contaminating hospital microorganisms provides the basis for
maintaining standards of hygiene (cleaning, disinfection,
sterilization) throughout the hospital as well as good
engineering and building [Ruef 2005].
Role of antibiotic treatment
At least 30% hospital patients receive antibiotics, and this exerts
strong selective pressure on the microbial flora, especially of
the gastrointestinal tract, leading to the development of
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antibiotic-associated diarrhoeas due to Clostridiulll difficile, one
of the commonest causes of outbreaks of hospital infection.
Sensitive species or strains of microorganisms which normally
maintain a protective function on the skin and other mucosal
surfaces tend to be eliminated, whereas those that are more
resistant survive and become endemic 111 the hospital
population [Ruef 2005]. This may restrict the range of agents
available for treatment and may lead to the transmission of
plasmid-mediated antimicrobic resistance into strains that show
increased virulence, survival and spread within the hospital
[Step han 2001].
Microorganisms causing hospital (leU) infection
Infection is a major cause of morbidity and mortality among
patients admitted in intensive care units (reUs). Each year,
health care associated infections affect an estimated two million
Americans, including 500,000 intensive care unit (ICU) patients,
resulting in an estimated 90,000 deaths and $4.5 billion 111
excess health care costs (NCID 2006). ICU patients are at
increased risk of acquiring infections, most of which are
associated with the use of invasive devices [Richards et al. 2000,
Benn 1985] such as:
:r Urinary tract infection associated with the use
of indwelling urinary catheters to drain the bladder [Saint
et. al. 2002].
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.,. Bloodstream infections associated with the use of central
line catheters, inserted in about one-half of reu patients to
provide medication, nutrition, and fluids (O'Grady et. a!.
2002).
.,. Pneumonia associated with the use of mechanical
ventilators in patients requiring assisted breathing (Tablan
et. a!. 2004).
The most important microorganisms responsible for hospital
infection are listed in Table 2.2 [Ducel 2002]:
Table 2.2 Nosocomial Infections Due to Bacteria and Fungi in
ICU Patients
Infections
Bloodstream Infections (BSI)
Lower respiratory tract infection / Ventilator associated pneumonia (V AP)
Urinary Tract Infection (UTI)
Upper respiratory tract infection
Gastrointestinal,
Surgical-site infections (SSI)
Nosocomial etiologies
Coagulase-negative Staphylococci, Enterococci, Fungi, Candida, StaphlflococclIs allrCIIS, Escherichia coli, Klcbsiella spp., PSeUdOl/lOllaS spp.
S trep tococcus pnCU1/101I iac, Hacl/lOph i IllS inflllellzac, Klebsiella pneul/lOlIiae, Lcgiollclla pne1l1l1ophila, Mycoplasma, Chlalllydia, Pseudol/lOnas acrugil1osa, A ci IIctobacte r [mul/lIm i i, S taphyl ocoeCllS allrcus, MycobacteriulII tu/Jerculosis Gram-negative enterics, Eschericllia coli, Protells I'lIlgaris, PseudO/llonas spp. Klebsiella PIICUl/lOllial', Fungi, Enterococci, StrcptococCllS pyogcnes, MRSA, Fungi, Candida, Neisseria IIIcnillgitides Co njllcbacleri u III d i pll Illcriae. Escherichia coli, Bactcroides fragilis, Enterococci, Anaerobes SlapllljlocOCCllS allrCllS, Pselidolllollas spp. Coagulase -ve Staphlflococci, Enterococci, fungi, Enterobacter species, and Escherichia coli,
47
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Routes of Transmission
The hospital offers many opportunities for the exchange of
microbes, many of which are harmless and a normal part of the
balance between man and his environment. For there to be
significant risk of infection, several factors including the right
susceptible host and the appropriate inocu lum of infecting
microorganisms, must be linked via an appropriate route of
transmission, Understanding of the sources and transmission
routes of hospital infection enables efforts to be concentrated in
more effective preventive measures [Ruef 2005, Damani 2003].
Common routes of transmission for different microorganisms
are shown in the Table 2.3.
Table 2.3 Main routes of transmission
Route
Contact transmission
Droplet transmission
Airborne transmission
Description
The most important and frequent mode of transmission of nosocomial infections.
Occurs when droplets are generated from the source person mainly during coughing, sneezing, and talking, and during the performance of certain procedures such as bronchoscopy. Transmission occurs when droplets containing germs from the infected person are propelled a short distance through the air and deposited on the host's body.
Occurs by dissemination of either airborne droplet nuclei (small-particle residue {S lIm or smaller in size} of evaporated droplets containing microorganisms that remain suspended in the air for long periods of time) or dust particles containing the infectious agent. Microorganisms carried in this manner can be dispersed widel\' bv air currents and may become inhaled by a
48
Route
Common vehicle transmission
Vector borne transmission
Review of Literature
Description
susceptible host within the same room or over a longer distance from the source patient, depending on environmental factors; therefore, special air handling and ventilation are required to prevent airborne transmission. Microorganisms transmi tted by airborne transmission include Legiollcl/n, Mlicobncterilllll tuberculosis and the rubeola and varicella viruses.
Applies to microorganisms transmitted to the host by contaminated items such as food, water, medications, devices, and equipment.
Occurs when vectors such as mosquitoes, flies, rats, and other vermin transmit microorganisllls.
Contact transmission is divided into two subgroups: direct
contact transmission and indirect-contact transmission (Table
2.4)
Table 2.4 Routes of contact transmission
Route Description
Direct- Involves a direct body surface-to-body surface contact and contact physical transfer of microorganisllls between a susceptible transmissi host and an infected or colonized person, such as occurs when on a person turns a patient, gives a patient a bath, or performs
other patient-care activities that require direct personal contact. Direct-contact transmission also can occur between two patients, with one serving as the source of the infectious microorganisms and the other as a susceptible host.
Indirect- Involves contact of a susceptible host with a contaminated contact intermediate object, usually inanimate, such as contaminated transmissi instruments, needles, or dressings, or contaminated gloves on that are not changed between patients. In addition, the
improper use of saline flush syringes, vials, and bags has been implicated in disease transmission in the US, even when healthcare workers had access to gloves, disposable needles, intravenous devices, and flushes.
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Self-infection and cross-infection
Self-infection may occur due to transfer into the wound of
staphylococci (or occasionally streptococci) carried in the
patient's nose and distributed over the skin, or of coliform
bacilli and anaerobes released from the bowel during surgery.
Alternatively, cross infection may result from staphylococci or
coliform bacilli derived from other patients or healthy staff
earners. The organisms may be transferred into the wound
during operation through the surgeon's punctured gloves or
moistened gown, on imperfectly sterilized surgical instruments
and materials or by air-borne theatre dust. Postoperatively,
organisms may be transferred in the ward from contaminated
bed-linen, by air-borne ward dust or in consequence of a faulty
wound dressing technique (Figure 2.2) [Damani 2003].
Self infection
Cross infection
Figure 2.4 Self and cross infection
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Of all the possible routes, by far the most likely in this example
is self-infection from patient's own bowl flora and is therefore
against this route that most specific preventive measures in
colorectal surgery are directed. Understanding of possible
sources of infection and the methods available to block
transmission to susceptible sites forms the basis of hospital
infection control [Benn 1985, Ruef 2005].
Cross-infection is more often caused by 'hospital' strains
selected for characteristics of antimicrobial resistance and
virulence. An important example at present is MRSA, which can
be easily identified by the microbiology laboratory, which
should have a system for alerting the infection control team
who need to collect the following basic epidemiological data .
." patient details
r the si te and extent of infection
the dates of admission, operative procedures, first
recognition of infection 1255 15
specimens and laboratory isolates and typing results
ward and staff details.
The clustering of cases according to a common surgical team or
location in the ward may suggest a common source and may be
the first firm indication of an outbreak of hospital infection.
Soon after admission to hospital, individuals commonly become
contaminated with the 'hospital flora'. This has been shown
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with Staphylococcus aureus in studies of patients before and
during hospital treatment. Patients who need to stay longer in
hospital, e.g. those requiring intensive care or the elderly are
less able to withstand infection and the risk of hospital infection
are greater [Weinstein 1998].
Prevention and control
The infection control policy
The establishment of an effective infection control organization
is the responsibility of good management of any hospital. There
will normally be two parts [Damani, 2003, Wenzel 2003]:
:>- The infection con trol committee is a m u I ti -discipl inary
group of individuals who meet to discuss current
surveillance data to formulate and update policies for the
whole hospital on matters having implications for
infection for infection control, and to manage outbreaks of
nosocomial infection [Wenzel 2003].
,. An infection control team of workers, which is headed by
the infection control doctor (usually the microbiologist),
to take day-to-day responsibility for this policy.
The function of this team include surveillance and control of
infection and monitoring of hygiene practices, advising the
infection control committee on matters of policy relating to the
prevention of infection and the education of all staff in the
microbiologically safe performance of procedures. The infection
control nurse is a key member of this team. Close working links
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between the microbiology laboratory, infection control nurse
and different clinical specialties and support services (including
sterile serVices, laundry, pharmacy and engineering) are
important to establish and maintain the infection control policy
and to ensure that it is rationally based on and that the
recommended procedures are practicable. Some of the control
measures 111 which the infection control team should be
involved are as follows [Wenzel 1997, Silvestr 1999, Wenzel
2003, Ruef 2005]
Sterilization
The provision of sterile instruments, dressings and fluids is of
fundamental importance in hospital practice. Sterilization by
heat in high-vacuum autoclaves has become accepted hospital
practice [Favero 1991].
The development of these sterilizers for processmg wrapped
goods facilitated the provision of a centralized service of sterile
supply to wards, complementing the existing theatre service.
The availability of a wide range of prepacked single-use items
(syringes, needles, catheters, and drainage bags) sterilized
commercially by g-irradiation or ethylene oxide has further
improved aseptic procedures and removed the need for
reprocessing items that are difficult to clean and therefore
impossible to sterilize [Wenzel 2003].
Most fluids for topical use or intravenous administration are
now prepared commercially or 111 regional units where
standards of quality control and efficiency for bulk processes
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are more readily achieved than 111 individual hospital
pharmacies.
Aseptic Techniques
The provision of sterile equipments will not prevent the spread
of infection if there is carelessness in its usc. Wherever possible,
no touch techniques must be used, coupled with strict personal
hygiene on the part of the operator. These routines and may be
modified as required for other procedures such as wound
dressing and insertion of intravenous catheters [Favero 1991,
Wenzel 2003, Damani, 2003].
Cleaning and Disinfection
The general hospital environment can be kept in good order by
attention to basic cleaning, waste disposal and laundry. The use
of chemical disinfectants for walls, floors, and furniture is
necessary only in special instances, such as spillages of body
fluids from patients with blood-borne virus infections. Ward
equipments such as bedpan washer / disinfectors and
dishwashers should be monitored to ensure reliable
performance, and cleaning materials such as mop heads and
cloths should be heat disinfected and stored dry after use
[Favero 1991]. Pre-cleaning of contaminated instruments and
equipments, preferably by means of an automatic washing
process with an ultrasonicator, is an essential step before
disinfection or sterilization [Wenzel 2003, Damani, 2003].
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Skin Disinfection and Antisepsis
The ease of acquisition and transfer of transient hospital
contaminants, particularly Gram-negative bacilli on the hands
of staff, is an important factor in the spread of hospital
infection. Thorough hand washing after any procedure
involving nursing care or close contact with the patient is
essential. Alcohol-based hand antiseptics or 'rubs' have been
introduced in wards where routine hand washing with water
and detergents is not practica ble [Damani, 2003]. Gloves may be
worn for dirty contact procedures, such as emptying a urinary
drainage bag or bed-pan, although it should not be forgotten
that the gloved hand may also become contaminated by
transient hospital flora.
Proced ures for pre-opera ti ve disinfection of the pa tient' s skin
and for surgical scrubs are mandatory within the operating
theatre. Dilute 'in-use' solutions of antiseptics may readily
become colonized with Gram-negative bacteria and should be
replaced regularly. Ideally, single-use preparations should be
used. Restriction should be placed on the indiscriminate use of
antiseptics and disinfectants by means of a disinfectant policy
agreed by pharmacists, microbiologists and key users, such as
theatre staff [Simmons 1990, Favero 1991].
Prophylactic antibiotics
Widespread and haphazard use of antibiotics hastens the
emergence of antibiotic-resistant bacteria, and increases both
the incidence of toxic side-effects and the cost of treatment.
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However, rational antibiotic prophylaxis plays an important
role in infection control. Specific indications include peri
operative prophylaxis in gastrointestinal and gynecological
surgery directed predominantly against anaerobic infection and
for patients known to have bacteriuria at the time of urological
surgery or instrumentation, directed against the urine isolate.
An antibiotic policy which limits the choice of broad-spectrum
agents is important both for prophylaxis and treatment [Singh
2000].
Protective clothing
Different activities within the hospital require different degrees
of protection to staff and patients. In operating theatres wearing
of sterile gloves, headgear and face masks minimizes the
shedding of microorganisms. The properties of fabrics available
of theater use have improved, and now include close-weave
ventile fabrics that are comfortable to wear and allow
evaporation of moisture. 'Total protection' of operating site
may be considered for certain high-risk clean surgery such as
hip replacements, during which the surgical team may wear
exhaust-ventilated suits and operate under conditions of ultra
clean laminar air flow [Wenzel 2003, Bearman 2006].
For many ward procedures in which there may be soiling, or for
simple barrier nursing of patients with communicable diseases,
plastic aprons, and gloves are used. Gloves, face masks and
goggles are also indicated for specific procedures when dental
. ----------
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procedures. These are sometimes referred to as 'universal
precautions.
Isolation
The isolation procedures should list facilities and procedures
needed to prevent the spread of specific infections to other
patients (source isolation) and to protect susceptible or
immunocompromized patients (protective isolation). Effective
isolation demands a highly disciplined approach by all staff to
ensure that none of the barriers to transmission (air-borne,
direct and indirect contact) are breached. are breached. Multi
bedded rooms may be used, and even wards converted during
hospital outbreaks, but the simplest solution wherever possible
is to use single rooms [Hospital Infection Control Practice
Advisory Committee 1996, Damani, 2003].
Cubicle isolation, by which the patient is nursed alone in a
room separated by a door and corridor from other patients,
confer a substantial measure of protection. Preferably, each
isolation room has its own toilet and washing facility. Clean,
filtered air is supplied to the room, which should be at negative
pressure (exhaust-ventilated) to the corridor for source isolation
or at positive pressure (pressure-ventilated) to the corridor, for
protective isolation. If, however, there is a small airlock
vestibule separating the room from the outer corridor, then
exhaust ventilation of the airlock will give effective isolation for
either situation. The vestibules or lobby should contain a wash
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basin and include space for gownmg and equipment [Wenzel
2003].
In some critical situations such as bone marrow transplantation
units, where air-borne contamination with environmental
fungal spores is a problem, the efficiency of air filtration may be
increased and laminar airflow maintained as a barrier around
the patient. Stringent isolation, such as plastic tent or 'Trexler'
isolator, is required only for patients with highly contagious
infections, such as those due to Lasa, Marburg and Ebola
viruses, who are nursed in a high-security isolation unit.
Hospital building and design
The routine maintenance of the hospital building is important,
ensurIng that surfaces wherever possible are smooth,
impervious and easy to clean. Major rebuilding works on or
near the hospital sites may generate dust containing fungal and
bacterial spores, with implications for specialized units serving
immunocompromized patients. Close communications with
works department and hospital administra tion are necessary to
co-ordinate any protective action. When a new hospital or
modification of existing building is planned, the infection
control team should be closely involved in discussing the plans.
In many countries, guidance on new building design exists to
minimize potential hospital-acquired infection. Areas requiring
special attention include operating theatres, kitchens, acute
wards, laboratories, and air-conditioning systems [Sakharkar
1998]. The risk of Legionnaires' disease is reduced by installing
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water supplies that circulate below 20°C for the cold and above
60°C for the hot circuit.
Equipment
Any object or item for clinical use should be assessed to
determine the appropriate method, frequency and site of
decontamination. Wherever possible, heat processes are
preferred, although this may be precluded for certain thermo
labile items such as fiber-optic endoscopes [Damani, 2003].
Personnel
An occu pa tional heal th service in hospi ta Is shou I d screen staff
before employment and offer appropriate immunization.
Hepatitis B vaccine should be given to all health care workers.
Those at special risk performing exposure prone procedures,
such as invasive surgery, should be screened for blood-borne
vauses. All staff (including medical students) should receive
occupational health advice and protection [Damani 2003]. Staff
who have contracted specific infections such as diarrhoea or
following needles tick injury should report and be screened if
necessary [Wenzel 2003, Ruef 2005].
Monitoring
Routine microbiological monitoring of the equipments is of
little benefit, although monitoring of the physical performance
of air-conditioning plants and machinery used for disinfection
and sterilization is essential. In the event of an outbreak of
hospital infection, more specific monitoring targeted at the
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known or likely causative microorganism should be considered
[Ruef 2005].
Microbiological screening of staff or patients is not undertaken
routinely, but it may be needed for specific purposes: to detect
carriers of MRSA and hepatitis viruses in those performing
some types of surgery or where transmission to patients has
occurred.
Surveillance and the role of the laboratory
The detection and characterization of hospital infection
incidents or outbreaks relay on laboratory data that alert the
infection control team to unusual cluster of infection, or to the
sporadic appearance of organisms that may present a particular
infection risk or management problem. This is sometimes
referred to as the 'alert organism' system. Bacterial typing
schemes and antibiograms are very important in this regard.
Regular visits to wards are also important to record data on
infected patients for whom specimens have not been received
and to respond to problems as they occur. Such visits also serve
to provide opportunities for practical teaching, which is another
important element of the infection control team's responsibility
[Wenzel 2003, Damani, 2003, Ruef 2005].
Efficacy of Infection Control
The evidence base in the literature for acceptable proof of
efficacy of infection control measures is scant. These include
sterilization, hand-washing, closed-drainage systems for
unnary catheters, intravenous catheter care, peri-operative
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antibiotic prohylaxis for contaminated wounds and techniques
for the care of equipment used in respiratory therapy. Isolation
techniques are assumed to be responsible as suggested by
experIence or inferences. Measures which are now considered
to be ineffective include the chemical disinfection of floors,
walls, sinks and routine environmental monitoring [Ruef 2005].
Effective surveillance and action by the infection control team
have been shown to reduce infection rates. One important role
of the team is to monitor compliance with practices known to be
effective and to eliminate the many rituals or less effective
practices which may even increase the incidence or cost of
cross-infection. As further advances occur in medical care and
limited health care resources are spread across hospital and
community needs, innovations in infection control will need to
be evaluated for efficacy and cost-effectiveness. With this
understanding it is possible that hospital infection can be
controlled and largely prevented. The dictum of Florence
Nightingale, made over a century ago, that 'the very first
requirement in a hospital is that it should do the sick no harm',
remains the goal [Wenzel 2003, Ruef 2005].
Summary
Hospitals should take a variety of steps to prevent nosocomial
infections, including [Damani 2003]:
.,.. Adopt an infection control program such as the one
sponsored by the U.s. Centers for Disease Control (CDC),
which includes quality control of procedures known to
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lead to infection, and a monitoring program to track
infection rates to see if they go up or down.
,.. Employ an infection control practitioner for every 200
beds.
>- Identify high-risk procedures and other possible sources
of infection.
>- Strict adherence to hand-washing rules by health care
workers and visitors to avoid passlllg infectious
microorganisms to or between hospitalized patients
>- Strict attention to aseptic (sterile) technique 111 the
performance of proced u res, inc! uding use of steri Ie
gowns, gloves, masks, and barriers
r Sterilization of all reusable equipment such as ventilators,
humidifiers, and any devices that come in contact with
the respiratory tract
>- Frequent changing of dressings for wounds and use of
antibacterial ointments under dressings
, Remove nasogastric (nose to stomach) and endotracheal
(mouth to stomach) tubes as soon as possible
, Use of an antibacterial-coated venous catheter that
destroys bacteria before they can get into the blood stream
, Prevent contact between respiratory secretions and health
care providers by using barriers and masks as needed
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, Use of silver alloy-coated unnary catheters that destroy
bacteria before they can migrate up into the bladder
r Limitations on the use and duration of high-risk
procedures such as urinary catheterization
" Isolation of patients with known infections
" Sterilization of medical instruments and equipment to
prevent contamination
" Reductions in the general use of antibiotics to encourage
better immune response in patients and reduce the
cultivation of resistant bacteria
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Review of Literature
Intensive care unit (leU)
Introduction
An intensive care unit (TCU), critical care unit (CCU), intensive
therapy unit or intensive treatment unit (ITU) is a specialized
section of a hospital that provides comprehensive and
continuous care for persons who are critically ill and who can
benefit from treatment [Gupta et. al. 2007, Sakharkar 1998].
During the Crimean War In 1854, Florence Nightingale felt the
necessity to separate seriously wounded soldiers from less
seriously wounded was observed. Thus, Nightingale reduced
mortality from 40% to 2% on the battlefield, creating the
concept of intensive care.
In response to a polio epidemic (where many patients required
constant ventilation and surveillance), Bjorn Ibsen established
the first intensive care unit in Copenhagen in 1953 [Eggimann &
Pittet (2001)]. Dr. William Mosenthal, a surgeon at the
Dartmouth-Hitchcock Medical Center [Vincent, et al. (1995),
pioneered the first application of this idea in the United States.
In the 1960s, the importance of cardiac arrhythmias as a source
of morbidity and mortality 111 mvocardial infarctions (Heart
Attacks) was recognized.
Thus, the development of intensive care units made the care for
more seriously sick patients possible. It allowed utilizing more
technically oriented tools to monitor and get information
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Review of Literature
instantly about any changes of the patient's physiological
parameters and developed new strategies to save life.
Purpose
The purpose of the intensive care unit (ICU) is simple even
though the practice is complex. Healthcare professionals who
work in the ICU provide around-the-clock intensive monitoring
and treatment of patients seven days a week. Patients are
generally admitted to an ICU if they are likely to benefit from
the level of care provided. Intensive care has been shown to
benefit patients who are severely ill and medically unstable
that is, they have a potentially life-threatening disease or
disorder (Figure 2.3).
Figure 2.5 A man recovering from quadruple bypass surgery
in an intensive care unit [Custom Medical Stock Photo].
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Although the criteria for admission to an reu are somewhat
controversial- excluding patients who are either too well or too
sick to benefi t from in tensi ve care - there are four
recommended priori ties that in tensi vists (clinicians who
specialize in critical illness care) use to decide this question.
These priorities include:
, Critically ill patients III a medically unstable state who
reqUIre an intensive level of care (monitoring and
treatment).
,. Patients requiring intensive monitoring who may also
require emergency interventions .
.,. Patients who are medically unstable or critically ill, and
who do not have much chance for recovery due to the
severity of their illness or traumatic injury.
, Patients who are generally not eligible for leu admission
because they are not expected to survive. Patients in this
fourth category require the approval of the director of the
reu program before ad mission.
Descri ption
reu care requires a multidisciplinary team that consists of but
IS not limited to intensivists; pharmacists and nurses;
respiratory care therapists; and other medical consultants from
a broad range of specialties including surgery, pediatrics, and
anesthesiology. The ideal reu will have a team representing as
many as 31 different health care professionals and practitioners
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Review of Literature
who assist in patient evaluation and treatment. The intensivist
will provide treatment management, diagnosis, interventions,
and individualized care for each patient recovering from severe
illness [Damani 2003].
ICUs are highly regulated departments, typically restricting or
limiting the number of visitors to the patient's immediate
family even during visiting hours. The patient usually has
several monitors attached to various parts of his or her body for
real-time evaluation of medical stability. The intensivist will
make periodic assessments of the patient's cardiac status,
breathing rate, urinary output, and blood levels for nutritional
and hormonal problems that may arise and require urgent
attention or treatment. Patients who are admitted to the rcu for
observation after surgery may have special requirements for
monitoring. These patients may have catheters placed to detect
hemodynamic (blood pressure) changes or require endotracheal
intubation to help their breathing, with the breathing tube
connected to a mechanical ventilator [Brilli et. al. 2001].
In addition to the intensivist's role in direct patient care, he or
she is usually the lead physician when multiple consultants are
involved 111 an intensive care program. The intensivist
coordinates the care provided by the consultants, which allows
for an integrated treatment approach to the patient.
Nursing care has an important role in an intensive care unit.
The nurse's role usually includes clinical assessment, diagnosis,
and an individualized plan of expected treatment outcomes for
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Review of Literature
each patient (implementation of treatment and patient
evaluation of results). The rcu pharmacist evaluates all drug
therapy, including dosage, route of administration, and
monitoring for signs of allergic reactions. In addition to
checking and supervlslllg all levels of medication
administration, the ICU pharmacist is also responsible for
enteral and parenteral nutrition (tube feeding) for patients who
cannot eat on their own. rcus also have respiratory care
therapists with specialized training in cardiorespiratory (heart
and,-n(r1g) care for critically ill patients. Respiratory therapists ~ .....
generally provide medications to help patients breathe as well
as' the care and support of mechanical ventilators. Respiratory
therapists also evaluate all respiratory therapy procedures to
maximize efficiency and cost-effectiveness.
Large medical centers may have more than one ICU. These
specialized intensive care units typically include a CCU
(coronary care unit); a pediatric ICU (PICU, dedicated to the
treatment of critically ill children); a newborn ICU or NICU, for
the care of premature and critically ill infants; and a surgical
ICU (SICU, dedicated to the treatment of postoperative
patients).
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