Download - Shreya modi
BIOCHEMICAL BASIS OF ANTIMICROBIAL ACTIVITY OF
ANTIBIOTICS
Guided & Checked By :Shreyas Bhatt sir
Shreya M. ModiMSc Sem- IIIRoll no.- 11
Content
Introduction
Introduction
Antibiotics are chemical molecules or compounds that specifically targets and kill cells. Not only antibacterial, but also antifungal, antiviral and also antineoplastic compounds are also classified as antibiotics.
Antibacterial action generally follows some of the mechanisms such as inhibition or regulation of enzymes involved in the synthesis of cell wall, nucleic acid synthesis and repair, or protein biosynthesis. Antibiotics target the cell functioning of rapidly dividing cells.
1. The target of an antibiotic can be present only in bacteria but not in the
eukaryotic host.
2. The target in bacteria is different from the homologous target in the
eukaryotic host.
Bases of antibiotic selectivity
Modern genomics provide a great tool for identifying targets of new selective
antibiotics
Selectivity of antibiotics is not ‘natural’
Natural antibiotics are weapons that bacteria or fungi use to compete with other microorganisms.
Selectivity is not a ‘natural’ feature of antibiotics.
Most of clinically-useful antibiotics are fortuitously selective antibacterials.
Many antibiotics are omni-potent and inhibit growth of a wide variety of organisms. Such
antibiotics can be developed into selective drugs through modification of their chemical structures.
Antibiotics are classified as bacteriostatic or bactericidal.
Bacteriostatic drugs make bacteria dormant, but do
not kill them. Most bacterial cells resume growth after
removal of the antibiotic (e.g. chloramphenicol)
Bactericidal drugs kill bacteria (e.g. ciprofloxacin)
Conti….
Antibiotics with a bactericidal mode of action are preferred, especially for treatment of immunocompromised
patients. The mode (static vs. cidal) of antibiotic action may differ for different pathogens and may depend on the drug
concentration.
The basis of bactericidal versus bacteriostatic effects is poorly understood but maybe related to the accumulation of reactive oxygen radicals in the bacterial cells upon treatment with bactericidal
drugs.
sulf
onam
ides
-la
ctam
s
am
inogly
cosi
des
192
0
194
2
194
7
194
9
195
2
195
8
196
2
200
0
tetr
acy
clin
e
macr
olid
es
gly
cop
epti
des
stre
pto
gra
min
slin
cosa
mid
es
After the golden era of the 1940s-1950s, the progress in antibiotic discovery has significantly slowed down until the year
2000Golden era in antibiotic discoveryNo principally new antibiotics
Growing resistance
linezo
liddapto
myci
n200
3
Growth phase
Conventional antibiotics
Penicillins Cephalosporins Carbapenems Quinolones Aminoglycosides Macrolides Tetracyclines
Nitrofurantoin, metronidazole, clindamycin, vancomycin, teicoplanin, cotrimoxazole, fusidic acid, etc
Isoniazid, pyrazinamide, ethambutol, rifampin, cycloserine, etc
Inhibition of cell wall synthesis
Some of the antibacterial compounds interfere with the cell wall synthesis by weakening the peptidoglycan structures in bacterial cell wall, by this integrity of bacterial cell wall structure weakens and eventually disrupts.
Mammalian cells only have plasma membrane so these antibiotics specifically target only bacterial cells. That is these antibiotics do not induce any negative effect on the host mammalian cells.
Cont …
Antibacterial compound β-lactam can be used against both Gram-positive and Gram-negative bacterial cells.
Vancomycin another antibacterial compound also prevents cell wall biosynthesis in bacterial cells by interfering with transglycosylases enzyme activity.
But this compound can be used effectively against Gram-positive bacteria, as it is unable to penetrate the outer cytoplasmic membrane of Gram-positive bacteria.
Antibiotics inhibiting cell wall synthesis
Name Producer organism
Chemical nature
Site of action
Penicillin P.NotatumP.Crysogenum
Β lactum Antibiotic
TranspeptidaseReaction
Cephalosporine Cephalosporium aeremonium
Β lactum Antibiotic
TranspeptidaseReaction
Cycloserine Streptomyces spp.
Analogue of alanine
Inhibit formation of Park’s nucleotide
Bacitracin B.Subtilis Peptide Phosphatase reaction in lipid cycle
Vancomycin Str.orientatis Glycopeptide Polymerization step
The first stage
The second stage
Bacitracin
The third stage
ß-lactam antibiotics.
Fosfomycin
Beta-Lactam Antibiotics
Beta-lactam antibiotics Penicilins Cephalosporins Carbapenems Monobactams
All β-lactam antibiotic agents contain a β-lactam nucleus in its molecular structure.
Core structure of penicillins (1) and cephalosporins (2). Beta-lactam ring in red.
Beta-Lactams. Common features
All beta-lactams: are bacteriocidal. have the same mechanism of antibacterial
action. have no activity against MRSA and
atypical bacteria (Legionella spp., Mycoplasma spp., Chlamidia spp.).
have the allergic cross-reaction. have the same modes of bacterial
resistance.
Beta-Lactams Mechanism of action
β-Lactam antibiotics are analogues of D-alanyl-D-alanine amino acid
residues
irreversible binding to the active site of penecillin-binding proteins
(PBPs)
Penicillin-binding proteins (PBPs), enzymes that catalyze the last steps
of peptidoglycan synthesis (cross-linking).
Conti….
Inhibition of the PBPs prevents the final crosslinking of the nascent peptidoglycan layer
disrupting bacterial cell (bactericidal effect)
Penicillins. Clinical use
Generation Example Clinical use
Natural penicillins Penicillin G Syphilis, rheumatic fever meningitis, tonsillitis, scarlet fever, endocarditis
Antistaphylococcal penicillins
Methicillin Mild and moderate staphylococcal infections
Extended-spectrum penicillins
AmpicillinAmoxicillin
Noncomplicated community-acquired infections (lower and upper respiratory tract infections, UTIs, skin and soft tissues)
Antipseudomonal penicillins
Carbenicillin P.aeruginosa infections
Inhibition of nucleic acid synthesis
Some antibiotics inhibit the action of enzyme RNA polymerase, hence interfere with RNA (ribonucleic acid) synthesis in the cells. Antibiotics such as asdoxorubicin andactinomycin D interfere with RNA biosynthesis in both bacterial cells as well as in mammalian cells. These compounds are used in treating rapidly growing tumor cells in cancer patients. Some of the examples are Doxorubicin hydrochloride, Levofloxacin, Irinotecan hydrochloride, Rifampcin
Penicillins Penicillin G
Still useful for a number of diseases (e.g. meningitis, syphilis)
Cloxacillin For MSSA infections
Ampicillin, amoxicillin Active vs. Gram-positive (not MSSA), Gram-
negative organisms Augmentin, Unasyn
Broad spectrum, covers Gram-positive, Gram-negative and anaerobes
Piperacillin, Tazocin, Timentin Are active vs. Pseudomonas
CephalosporinsGeneration Example Spectrum
First Generation
Cefazolin Most active against gram-positive bacteria (staphylococci). Have no activity against gram-negative bacteria.
Second Generation
Cefuroxim Enhanced activity against gram-positive and some gram-negative bacteria.
Third Generation
Cefotaxime Broad-spectrum (gram-positive and gram-negative). Resistant to most type of beta-lactamases.
Fourth Generation
Cefepime Most active against gram-negative bacteria. Very active against P.aeruginosa. Resistant to beta-lactamases. Have little gram-positive activity.
Carbapenems
Imipenem Broad spectrum, covers Gram-positive,
Gram-negative (including ESBL-producing strains), Pseudomonas and anaerobes
Meropenem Less seizure-inducing potential, can be
used to treat CNS infections Ertapenem
Lacks activity vs. Acinetobacter and Pseudomonas
Has limited activity against penicillin-resistant pneumococci
Glycopeptide antibiotics Vancomycine
Is not absorbed from the gut.
IV administration. Excreted unchanged
by the kidneys.
VancomycinMechanism of action
Forms a complex with the C-terminal D-alanine of peptidoglycan precursors
Prevents the following addition of new units to the peptidoglycan
Inhibition of peptidoglycan synthesis
Bactericidal effect
Vancomycin activity
Do not penetrates the membrane of gram-negative organisms.
Gram positive organisms only Staphylococcus spp. including
Methicillin-resistant Staphylococcus aureus (MRSA)
Streptococcus spp. Enterococcus faecalis and E. faecium
Clostridium difficile and other Clostridia (cause pseudomembranous colitis)
Vancomycin. Clinical use
Serious, life-threatening gram-positive infections
MRSA infections
Pseudomembranous colitis caused by Clostridium difficile (oral administration of vancomycin)
VancomycinAdverse reactions
Nephrotoxity: mostly in combinations with aminoglycosides
Ototoxicity Red man syndrome (or red
neck syndrome): within 4–10 minutes after
the start of infusion flushing and an
erythematous rash at the face, neck and upper body.
is due to non-specific mast cell degranulation. It is not allergic reaction.
Quinolones
Ciprofloxacin Active vs. MSSA, Gram-negative and
Pseudomonas Levofloxacin
Has activity vs. Streptococcus pneumoniae, but slightly less active towards Pseudomonas compared to ciprofloxacin
Moxifloxacin Has activity vs. anaerobes but less active
towards Pseudomonas
Aminoglycosides
Active vs. some Gram-positive and Gram-negative organisms
Gentamicin Active vs. Pseudomonas
Tobramycin More active vs. Pseudomonas than gentamicin Shows less activity against certain other Gram-negative
bacteria Amikacin
More stable to enzymes, used in severe infections by gentamicin-resistant organisms
Streptomycin Used for tuberculosis
Macrolides
Erythromycin Active vs. Gram-positive organisms, atypicals GI side effects
Clarithromycin Slightly greater activity than erythromycin
Azithromycin Slightly less active than erythromycin vs.
Gram-positive but enhanced activity vs. some Gram-negative organisms
Tetracyclines
Drug of choice in infections caused by Chlamydia, Rickettsia, Brucella and Lyme disease
Value has decreased due to increasing bacterial resistance
Tetracycline Role in Helicobacter pylori eradication (less
frequently used than other antibiotics) Doxycycline
Once daily Minocycline
Broader spectrum
Other antibiotics
Clindamycin Vs. Gram-positive cocci and anaerobes
Metronidazole Vs. anaerobes Preferred therapy in antibiotic associated
diarrhoea (Clostridium difficile) than oral vancomycin, although unlicenced
Vancomycin, teicoplanin For Gram-positive organisms (including MRSA)
Other antibiotics
Cotrimoxazole Role in uncomplicated UTI, UTI prophylaxis,
acute exacerbations of chronic bronchitis Pneumocystis carinii (now jiroveci) infections
Nitrofurantoin For UTI, prophylaxis vs. UTI
Fusidic acid, rifampin For penicillin-resistant staphylococci Not for monotherapy due to risk of emergence
of resistance
Mechanisms of Action of Antibacterial Drugs
Inhibition of protein synthesis Structure of prokaryotic ribosome acts as
target for many antimicrobials of this class Differences in prokaryotic and eukaryotic
ribosomes responsible for selective toxicity Drugs of this class include
Aminoglycosides Tetracyclins Macrolids Chloramphenicol Lincosamides Oxazolidinones Streptogramins
Inhibition of nucleic acid synthesis
RNA, which participate in the protein biosynthesis.
DNA, which carries the entire genetic information for the characters to be expressed by the organisms, by acting as hereditary material.
Inhibition of RNA synthesis
Certain antibiotics are able to bind with the key enzyme involved in RNA synthesis like-
RNA polymerese. Binding of antibiotics to this enzyme
interferes with the functioning of this enzyme and prevent RNA synthesis.
Certain other antibiotics bind with GC pair of DNA and prevent unfolding of DNA, required for transcription. thus, they inhibit RNA synthesis.
E.g.- Mitomycin C Actinomycin D
Antibiotic Mode of action
Actinomycin D
Binds to GC pair of DNA and interferes with transcription And replication process.
Mitomycin C
Binds to GC pair of DNA and interferes with transcription And replication process.
Rifampicin Binds with β- subunit of bacterial RNA polymerese andInterferes with transcriptional process.
Rifamycin Binds with β- subunit of bacterial RNA polymerese andInterferes with transcriptional process.
Griseofulvin Binds to DNA polymerese
Anthramycin group
Binds to DNA and damage its structure and function.
Inhibition of protein synthesis
Protein synthesis is a multi-step process. Majority of antibiotics inhibit the process s that occurs in the 30S 0r 50S subunit of 70S bacterial ribosome, this in turn inhibits the protein biosynthesis.
Most of the antibiotics inhibits the formation of 30S initiation complex or altogether inhibits the formation of 70S ribosome by the 30S and 50S ribosome subunits or they inhibit assembling of amino acids into a polypeptide chain.
Tetracyclines, includingdoxycycline, block protein synthesis by preventing the binding of aminoacyl- tRNA in 30S ribosome subunit. These compounds block protein synthesis in both prokaryotic and eukaryotic system.
Streptomycin interferes with the formation of 30S initiation complex hence inhibits the protein biosynthesis. Erythromycin interferes with the assembly of 50S subunit of ribosome hence inhibit the protein synthesis.
Antibiotics lincomycin and clindamycin inhibits enzyme peptidyl transferase, hence prevent the protein synthesis.
Conti….
Whereas antibiotic puramycin does not inhibits the enzymatic process, but they act as an analoge of 3'-terminal end of aminoacyl-tRNA, hence disrupts protein synthesis and causes premature polypeptide chain termination. In other words this antibiotic produces non functional proteins in the cell.
Some of the examples for this category of antibiotics are Doxocycline hyclate, Erythromycin, Hygromycin B, Kanamycin disulfate salt and much more.
Name Chemical nature Target site of action
Puromycin Structural analogue
Compete with binding of tRNA aminoacyl tRNA at a side on ribosome.
Streptomycin Aminoglycoside Binds to 30s ribosomal subunit and cause misreading of codons.
Tetracycline Naphthalene ring structure
Binds to 30s ribosomal subunit and prevent binding of aminoacyl tRNA to ribosome
Chloramphenicol Nitrobenzene Ring
Binds to 50s ribosomal subunit and interferes with peptide bond formation.
Erythromycin Macrolide ring Binds to 50s ribosomal subunit and interferes with peptide bond formation as well as block translocation step.
Good news vs. bad news Good news
A few novel antibiotics have shown promising results / are undergoing clinical studies
Bad news As immunosuppressive diseases and use of
immunosuppressive agents become more prevalent, opportunistic infections becomes more common, esp. by organisms rarely encountered previously Diseases: e.g. HIV, leukemia Drugs: e.g. in solid organ transplants, bone
marrow transplants, rheumatoid disorders Development of bacterial resistance to
antibiotics is much faster than research and development of new antibiotics
Conclusion
Antibiotics inhibits the growth of infectious agents such as bacteria, virus, fungus or other types of microorganisms by inhibiting cell wall formation or nucleic acid synthesis or protein synthesis.
References
Thank you..