infection. virulence factors. bacterial...
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Infection.Virulence factors.
Bacterial pathogenesis.Bacterial pathogenesis.(Prepared by Inzhevatkina S.M., Department of
Microbiology and Virology of of
Russian National Research
Medical University NI Pirogov)
Infection and Infectious Disease
Infection is the entry, establishment, and
multiplication of a pathogenic microorganisms
within a host.
Infection doesn’t invariably result in disease.
In fact, disease is but a rare consequence of In fact, disease is but a rare consequence of
infection, which is a common natural event.
Infectious disease is the state of damage or
toxicity in the body caused by an infectious
agent, when the host’s body cannot carry its
normal function because of the presence of
infectious agent or its products.
Koch’s PostulatesSometimes it is difficult to prove that a specific
bacterial species is the reason of a particular disease.
In 1884 Robert Koch proposed the basic postulates, which are considered to be applied broadly to link many specific bacterial species with particular disease:
1. The microorganism should be found in all cases of the disease in question, and its distribution in the body should be in accordance with the lesions observed.should be in accordance with the lesions observed.
2. The microorganism should be grown in pure culture in vitro for several generations.
3. When such a pure culture is inoculated into susceptible animal species, the typical disease must result.
4. The microorganism must again be isolated from the lesion of such experimentally produced disease and identified as original specific causative agent
Koch’s Postulates
Limitations of Koch’s Postulates Koch’s postulates are not universal. There are
several exceptions in several cases from the basic rules:
1. Existence of bacteriocarriers abandoned the first postulate (e.g., cholera bacteriocarriers).
2. Some causative agents, such as Treponema 2. Some causative agents, such as Treponema pallidum, can’t be grown in vitro; Rickettsia and Chlamydia spp., viruses, prions never can be cultivated on artificial culture media;
3. Some causative agents, such as Streptococcus pyogenes, can cause several different diseases.
Limitations of Koch’s Postulates (continuation)
4. Neisseria gonorrhoeae, Bordetella pertussis,
Corynebacterium diphtheriae, Salmonella Typhi,
Vibrio cholerae, etc. can’t be inoculated into
animal model because they cause antroponotic animal model because they cause antroponotic
infection (source of infection is only human
being);
5. Mixed infection (especially wound
inflammatory diseases, pneumonia, nephritis)
can be caused by several (not particular)
pathogens simultaneously.
Characteristic Features of Infectious Disease
1. Presence of the causative agent.
2.Specificity. Each causative agent can cause the
particular disease. But sometimes exceptions can occur:
Different pathogens can cause infection with the same
clinical manifestations or one and the same pathogen can
cause clinically different forms of infection.
3. Infectivity is the ability of pathogen to enter, survive,
multiply and cause disease in a susceptible host.
4. Presence of incubation period.
5. Cyclic development.
6. Formation of postinfectious immunity.
Stages of Typical Infectious Disease
1. The incubation period, which is the time between the acquisition of the organism and the beginning of clinical manifestation;
2. The prodromal period, during which prodromal period, during which nonspecific symptoms such as fever, malaise, and loss of appetite occur;
3. The specific illness period (period of main clinical manifestations);
4. The outcome, which can be different (recovery (convalescence), chronic disease, bacteriocarrier state or death).
The specificity of western classification for stages of infectious
disease: they distinguish the specific illness period (period of
main clinical manifestations) as two separate stages
11 2 3 4
outcome
Classification of Microorganisms,
Based on Their Relationships
to the Hosts Saprophytes are free living organisms, which
live on decaying organic matter. They fail to multiply on living tissue and so are not important in infectious disease. in infectious disease.
Parasites are organisms that can establish themselves and multiply in hosts.
Parasites may be commensals or pathogens. Commensal microbes can live in a host
without causing any disease.
Pathogens are those which are capable of produce disease in a host.
Classification
of Bacterial PathogensBacterial pathogens can be classified into two
broad groups, primary (true) pathogens andopportunists.
Primary pathogens are capable of establishing infection and causing disease in previously healthy individuals with intact immunological defenses. individuals with intact immunological defenses. However, these bacteria may more readily cause disease in individuals with impaired defenses.
Opportunistic bacteria cause disease only in people with preexisting diseases; disruptions in protective barriers of the skin and mucous membranes, leading to penetration in unprotected sites; defects of immune system, etc. that enhance their susceptibility(e.g., Pseudomonas aeruginosainfect burn patients and the lungs of patients with cystic fibrosis, coagulase-negative staphylococci cause persistent bacteriemia in patients with artificial heart valves, long-dwelling catheters and shunts) .
Classification of PathogensPathogens can be classified in
Strict intracellular pathogens, which can not cannot be cultivated in broth medium and can replicate only in vivo or in tissue-cultured cells in vitro (Chlamydia and Rickettsia).
Facultative intracellular pathogens can be Facultative intracellular pathogens can be grown in vitro on culture media and used to infect tissue culture (M.tuberculosis, M.bovis, B.melitensis, F.tularensis, S.Typhi, S.dysenteriae, L.pneumophila, Yersinia pestis, Listeria monocytogenes, etc.).
Extracellular pathogens are B.pertussis, B.burgdorferi, T.pallidum, C.jejuni, C.diphtheriae, L.interrogans, H.ducrey etc.
Classification of Infections by RepetitionInitial infection with organism in host constitutes
primary infection.
Recurrence is repetition of the same infection after convalescence due the remaining pathogen in the host’s body.
Subsequent infection by the same organism in a host is called reinfection.
Superinfection is subsequent infection with the same pathogen from the external source before convalescence (syphilis).
When in a host whose resistance is lowered by preexisting infectious disease, a new organism may set up the secondary infection. Usually opportunistic infections are viewed as secondary infections (because immunodeficient state or injury are the predisposing factors).
Classification of Infections by Localization
Infection is characterized by localization in
generalized (the pathogen disseminated in the whole host body), the whole host body),
localized (e.g., appendix or tonsils are occupied by microorganism) or
focal process (deep-seated process, e.g. abscesses).
Classification of Infections by SourceDepending on whether the source of infection is from the
host’s own body or from external sources, infections are classified as endogenous and exogenous, respectively.
• Exogenous infection can be– Antroponotic (from humans): diphtheria, whooping cough, rubella,
mumps, measles, etc.– Zoonotic (from animals); plague, anthrax, brucellosis, rabies, tick-
borne encephalitis, Q fever, etc.– Sapronotic (from environment, e.g. soil, water): legionellosis, – Sapronotic (from environment, e.g. soil, water): legionellosis,
tetanus, etc.
Cross infection is termed to the situation, when a patient suffering from a disease and new infection is set up from another host or external source.
Nosocomial infections are termed cross infections occurred in hospital.
The term iatrogenic infection refers to physician induced infections resulting from investigative, theurapetic or other procedures.
Classification of Infections
by Presence and Expression of
Clinical Manifestations
• Typical (presence of typical clinical manifestations of the disease)manifestations of the disease)
• Atypical (typical clinical manifestations of the disease are not present)
• Latent,or inapparant (clinical symptoms of the disease are not apparent)
Classification of Bacteriocarriers
Carrier state is the state of being a carrier of pathogens, but the infected host lacks the infectious disease.
Type of carriage– healthy (never suffered from the disease)
– convalescent (after recovery from disease)(after recovery from disease)
Duration of carriage:
a. temporary (<6 months)
b. chronic (several years)
Type of transmission of carriage:
a. contact
b. paradoxic (acquired pathogen from another carrier)
Transmission of PathogensPathogens are transmitted by direct contact (plague,
anthrax, tularemia), indirect contact (fomites in the transmission of diphtheria and trachoma), inhalation (influenza, parainfluenza, tuberculosis, whooping cough, measles, mumps, adenoviral infection), ingestion (cholera, food poisoning, dysentery), inoculation (tetanus, rabies), sexual transmission (gonorrhea, urogenital chlamydial sexual transmission (gonorrhea, urogenital chlamydial disease, syphilis), transfusion of blood and transplantation (HIV-infection, hepatitis B and C), insects (biological vectors(tick-borne encephalitis, plague), congenital (syphilis, rubella and cytomegaloviral infection). Iatrogenic infection can be a result of exchange transfusion, dialysis, heart and transplantant surgery, injections, lumbar puncture and catheterization.
Route of Infection
Route of infection can be different. Some
bacteria, such as streptococci, can initiate
infection whatever be the mode of entry. Others
can survive and multiply only when introduced
by the optimal routes. Cholerae vibrios are
effective orally but are unable to cause infection
when introduced subcutaneously. This
difference is probably related to modes by which
different bacteria are able to initiate tissue
damage and establish themselves.
Spreading of the PathogenSpreading of the pathogen in the host’s body
can be characterized by
toxinemia (circulation of toxins in bloodstream), bacteremia (circulation of bacteria in bloodstream),
septicemia (circulation and multiplication of bacteria in bloodstream, formation of toxic bacteria in bloodstream, formation of toxic products and induction of high, swinging type of fever),
septicopyemia (septicemia accompanied by abscesses in internal organs, e.g. spleen, liver and kidney).
Continuation
(Duration of Infection)
• 1. Acute
• 2. Subacute (subclinical)
• 3. Chronic3. Chronic
• Western authors distinguish also fulminant
development of infection
Classification of Spreading of Infectious Disease in the Community
Endemic diseases are those which are constantly present in a particular area.
An epidemic disease is one that spreads An epidemic disease is one that spreads rapidly, involving many persons in an area at the same time.
A pandemic is an epidemic that spreads through many areas of the world involving very large numbers of persons within a short period.
Pathogenicity and VirulencePathogens should be able to enter the body, multiply in
the tissue, damage the tissue and be capable to resist the host defenсe.
Pathogenicity is referred to the ability of microbial species to produce disease.
Virulence is referred to the ability of microbial strains to produce disease. Vaccine strains are avirulent but enhancement of virulence is known as exaltation and can be demonstrated experimentally by serial passages in susceptible hosts. Reduction of virulence is known as attenuation and can be achieved by passage through unfavorable hosts, repeated cultures in artificial media etc. Thus virulence is the measure of pathogenicity.
Virulence is the sum of invasiveness, toxigenicity, adhesive and antiphagocytic factors.
Virulence FactorsVirulent bacteria (pathogens) have mechanisms
that promote their growth in the host at the expense of
the host and tissue functions. The virulence factors
(factors of pathogenicity) are divided in:
• adhesive factors (adhesins that promote
colonization, IgA proteases) colonization, IgA proteases)
• antiphagocytic factors (capsule and antiphagocytic
surface proteins)
• invasive factors (degradative enzymes which
facilitate tissue invasion),
• toxins (endotoxins and exotoxins).
Problems in Classification of Some Virulence Factors
Some virulence factors possess multiple activities (simultaneously several virulent properties) or difficult to classify by the given widely accepted classification the given widely accepted classification (e.g., byproducts of fermentation (acids, gases, and another substances), ureases, β-lactamases, caratinoid pigments, peroxidase) .
Colonization and AdhesionColonization of the sites which are normally
sterile implies the defect in a natural defense
mechanism or a new portal of entry. The result
of colonization is establishment of stable
population of bacteria in the host. Such focus of
infection serves as a source for spreading of infection serves as a source for spreading of
pathogen to other tissues.
Adhesion is necessary to avoid innate host
defense mechanism (e.g., peristalsis of the
gut, flushing action of the mucus, saliva and
urine). For invasive bacteria, adhesion is an
essential preliminary stage before penetration
in the mucus.
Adhesion
The presence and type of adhesins (usually proteins)
depends on the strain of pathogen for the particular
receptors (usually carbohydrates) on the mammalian cell
membrane of tissue or organ.
Adherence of the m/os is dependent on 3 important
characteristics; characteristics;
1. the bacteria’s own adhesive characteristics
a. nonspecific properties: (e.g., surface charge,
hydrophobicity of m/o),
b. specific structures (complementary adhesins for specific
receptors),
2. the receptive features of the target tissue,
3. the fluid which can be present between both surfaces.
Scheme of Adhesin-Receptor Interaction
Examples of Adhesion
Classification of AdhesinsAdhesins are divided on
1. fimbria, or pili (numerous thin, rigid rod-like
structures, 7 nm in diameter, with a central hole
running along the lenth);
2. non-fimbrial adhesins (protein or polyssacharide
structures on bacterial cells);structures on bacterial cells);
3. flagella of several pathogens are considered to
be adhesins (V.cholerae, Campylobacter jejuni).
4. IgA-protease degrades immunoglobulin A and
improves colonization and adhesion on mucus
surface (Haemophilus influenzae, Neisseria
meningitidis, Neisseria gonorrhoeae, Streptococcus
pneumoniae).
Classification of Adhesins
Adhesion on Tooth
Fimbrial AdhesinsMany of adhesin protein are present on tips of
fimbriae (pili), and binds tightly to specific sugars on target
tissue.
E.g., different virulent strains of E.coli possess
different adhesins:
CFA-I and CFA-II are present on enteropathogenic strains CFA-I and CFA-II are present on enteropathogenic strains
of E.coli causing diarrhea in humans;
K88 are present on enteropathogenic strains of E.coli
causing diarrhea in piglets and pigs;
X- (S, M) and P-adhesins are typical for
puelonephritogenic E.coli.
Pili are typical for Neisseria, Pseudomonas,
Bacteroides, and Vibrio spp.
Examples of Non-fimbrial Adhesins
Teichoic acids of coagulase-negative staphylococci adhere to prostetic devices and catheters,
Lipoteichoic acids of Streptococcus pyogenes bind fibronectin,
Filamentous haemagglutinin of Bordetella Filamentous haemagglutinin of Bordetella pertussis – to respiratory epithelium,
Mannose-resistant haemagglutinin of Salmonella enterica serotype Typhimurium –to mucus (Salmonella Typhimurium),
Fibronectin-binding proteins of Staphylococcus aureus and Treponema pallidum – to mucus,
etc.
Flagella are motility factor which are also
considered to be responsible for virulence of
the microorganism
Electron Microscopy
of Salmonellae
Vibrios are highly motile curved rods
with a single polar flagellum
Electron microscopy.
Antiphagocytic factorsI. Capsule reduces efficiency of phagocytosis by several
mechanisms:
1. Carbohydrate nature of capsule ensures weakly immunogenic properties, thus, prevent efficient opsonization by antibodies and complement;
2. Capsule protects m/os from destruction within the phagolysosomeinside macrophage or microphage.inside macrophage or microphage.
3. A biofilm, made from capsular material, prevents antibody and complement from getting to m/o.
Also capsule increases adhesive properties of most
m/o. Loss of ability to form capsule leads to formation of
avirulent strain for several pathogens.
II. Plasmacoagulase of Staphylococcus aureus converts
fibrinogen to fibrin which causes clotting and walling of the
site of infection.
Encapsulated microorganisms• Staphylococcus aureus
• Streptococcus pneumoniae
• Streptococcus pyogenes (group A)
• Streptococcus agalactiae (group B)
• Bacillus anthracis
• Bacillus subtilis
• Clostridium perfringens
• Neisseria gonorrhoeae• Neisseria gonorrhoeae
• Neisseria meningitidis
• Haemophilus influenzae
• Escherichia coli
• Klebsiella pneumoniae
• Salmonella spp.
• Yersinia pestis
• Campylobacter fetus
• Bacteroides fragilis
• Cryptococcus neoformans (eukaryoitic m/os – yeasts)
Streptococcus pneumoniae in sputum (by Gram
stain). G+ diplococci with capsule (colorless zone
around m/os)
Capsule of B.anthracis in tissue
(methylene blue)
Capsule of Y.pestis in tissue
(fuchsin)
Plasmacoagulase TestCoagulase is an enzyme that clots blood plasma. This test is
performed on G+ m/os, e.g.,to identify the coagulase positive
Staphylococcus aureus. Coagulase is a virulence factor of
S.aureus. The formation of clot around an infection caused by
this bacteria protects it from phagocytosis. This test
differentiates S.aureus from other coagulase negative
Staphylococcus spp.
Invasive FactorsThe invasion of m/os is achieved by
secretion of bacterial extracellular substances. They are designated as invasins. Most invasive factors are degradative enzymes that locally break degradative enzymes that locally break down tissue, thereby providing nutrition and promoting spreading of pathogen. Several known invasins are toxins. The invasive activity of m/os is part of the basis of pathogenesis of most infectious diseases.
Invasins (“Spreading Factors”)
"Spreading Factors" is a term for designation for a family of bacterial enzymes which disrupt the host cells of tissue and intracellular spaces.
Hyaluronidase is the enzyme which is depolimerazing hyaluronic acid, which is the depolimerazing hyaluronic acid, which is the interstitial cement of connective tissue. The enzyme is produced by staphylococci, streptococci, and clostridia.
Collagenase breaks down collagen, the framework of muscles. The enzyme is produced by staphylococci and clostridia (C.histolyticumand C.perfringens).
Invasive Factors
Invasins (“Spreading Factors”)(continuation)
Neuraminidase degrades neuraminic acid (sialic acid), an intercellular cement of the intestinal mucosa. Its function is to cleave a sialic acid residue off ganglioside-GM1 (which modulates cell surface and receptor activity), turning it into asialo-GM1 to which attachment factor (pili) bind preferentially The which attachment factor (pili) bind preferentially. The enzyme is produced by Vibrio cholerae, Pseudomonas aeruginosa, Bacteroides fragilis, Mycoplasma pneumoniae, Corynebacterium diphtheriae, and Shigella dysenteriae.
Fibrinolysin ( designated as streptokinase for streptococci and staphylokinase for staphylococci) convert inactive plasminogen to plasmin which splits fibrin and prevents blood clotting.
The Enzymes which Cause Cell LysisThe universal view on these enzymes is absent. Such
enzymes disrupt animal cell membrane, by insertion into the membrane and formation pores, leading to cell lysis, or by splitting the phospholipids, components of cytoplasmic cell membrane.
Phospholipases (e.g. , α-toxin of Clostridium perfringens) hydrolyze phospholipids in cell membranes by removal of polar head groups.
Lecithinases destroy lecithin (phosphatidylcholine) in cell Lecithinases destroy lecithin (phosphatidylcholine) in cell membranes. They are produced by Staphylococcus aureus and Clostridium perfringens.
Hemolysins, are channel-forming proteins in membranes of erythrocytes and other cells (e.g., phagocytes). They are produced by staphylococci, streptococci, and clostridia.
Leukocidins disrupt neutrophil membranes and causes discharge of lysosomal granules. They are produced by staphylococci and streptococci (streptolysins). These latter 2 enzymes are considered to be bacterial exotoxins.
Clostridium sordellii and Clostridium perfringenscolonies growing on Egg Yolk Agar. Opaque zones
around colonies are vividly seen (positive lecithinase).
HemolysinsA:β-hemolysis; B:α-hemolysis; C:γ-hemolysis
Invasins with Multiply Functions
Some bacteria can secrete a wide variety of extracellular enzymes (e.g., proteases, lipases, glycohydrolases,mucinases, nucleases (DNA-ases and mucinases, nucleases (DNA-ases and RNA-ases), etc.). These enzymes have many functions. They participate in invasion directly or indirectly, but also play role in bacterial metabolism and nutrition.
Special complex systems ensuring adhesion and invasion
Special complex systems ensuring adhesion and
invasion had been developed by representatives of
Enterobacteriaceae family (e.g., Salmonella,
Shigella, Yersinia, E.coli) and Listeria
monocytogenes. monocytogenes.
Salmonella uses the device to promote its
uptake into a vesicle and live intracellularly in the
macrophages. Shigella uses a type III secretion
device that resembles molecular syringe for pore
formation to enter the cells, and inside the cells , the
m/os causes actin polimerization and push bacteria
in to the adjacent cells.
ToxinsToxins are bacterial products that directly
harm tissue or trigger destructive biological activities.
Toxins and toxin-like activities are degradative enzymes that cause lysis of the cells (thus, they are also invasins) or specificcells (thus, they are also invasins) or specific-binding proteins that initiate toxic reactions in a specific target tissue. Some toxin cause inappropriate or excessive stimulation of innate and adaptive immune systems.
Toxins are classified into exotoxins and endotoxins.
EndotoxinsEndotoxins are part of the outer membrane of
cell wall of G- bacteria (of the envelope of G-bacteria),such as E. coli, Vibrio cholerae, Pseudomonas aeruginosa, Salmonella, Shigella,Proteus, Klebsiella, Brucella, Neisseria, Haemophilus, Bacteroides, Prevotella, Porphyromonas, etc. Lipopolysaccharide (LPS) has many functions, which are essential for bacterial many functions, which are essential for bacterial survival and growth (Lecture “Morphology of microorganisms”). The biological activity of endotoxin is associated with the lipopolysaccharide (LPS). Toxicity is associated with the lipid component (Lipid A) and immunogenicity (antigenicity) is associated with the polysaccharide components. The cell wall antigens (O antigens) of G- bacteria are components of LPS.
Localization of Lipopolysaccharide in cell wall of G- microorganism
Structure of LPS
Main Physiological and Pathological effects
of Endotoxin
1. Fever (see next scheme)2. Leukopenia occurs early with onset of fever.3. Hypoglycaemia occurs because of enhancement of glycolysis in many cell types of the host.
4. Hypotension.5. “Endotoxic” or “septic” shock may develop in severe G- bacteraemia.6. Activation of complement system by alternative 6. Activation of complement system by alternative pathway.7. Disseminated Intravascular Coagulation (DIC). DIC may occur in G- bacteraemia. It is initiated on activation of factor XII (Hageman factor) of coagulation cascade by endotoxin which finally leads to conversion of fibrinogen to fibrin. Endotoxin leads platelets to adhere on vascular endothelium. Endotoxin can activate plasminogen to plasmin which acts on fibrin producing fibrin-split products.
8. Trombocytopenia.
Mechanism of Fever, Caused by
Bacterial Endotoxin
Endotoxins
For the most part, endotoxins remain
associated with the cell wall of G- bacteria until
disintegration of the bacteria. In vivo, this results
from autolysis, external lysis, and phagocytic
digestion of bacterial cells. But small amounts of digestion of bacterial cells. But small amounts of
endotoxin may be released in a soluble form,
especially by young cultures.
LPS activates complement by the alternative
(properdin) pathway and may be a part of the
pathology of most Gram-negative bacterial
infections.
EndotoxinsCompared to the classic exotoxins of
bacteria, endotoxins are less potent and less specific in their action, since they do not act enzymatically.
Endotoxins are heat stable (boiling for 30 Endotoxins are heat stable (boiling for 30 minutes does not destabilize endotoxin), but certain powerful oxidizing agents such as , superoxide, peroxide and hypochlorite degrade them.
Endotoxins are weakly antigenic, they cannot be converted to toxoids.
Comparison of
exotoxins and endotoxins (summary)
ExotoxinsExotoxins are proteins.
They are typically soluble proteins secreted by living bacteria during exponential growth. The production of protein toxins is generally specific to a particular bacterial species (e.g. only Clostridium botulinum produces botulinum toxin; only Corynebacterium diphtheriae produces the diphtheria toxin). toxin).
Usually, virulent strains of the bacterium produce the toxin (or range of toxins) while nonvirulent strains do not, such that the toxin is the major determinant of virulence.
Both G+ and G- bacteria produce soluble protein toxins.
Bacterial protein toxins are the most potent poisons known and may show activity at very high dilutions.
ExotoxinsProtein toxins resemble enzymes because:
They are proteins.
They are denatured by heat, acid, proteolytic enzymes.
They have a high biological activity (most act catalytically).
They exhibit specificity of action. Such bacterial They exhibit specificity of action. Such bacterial protein toxins are highly specific in the substrate utilized and in their mode of action. The substrate (in the host) may be a component of tissue cells, organs, or body fluid. Usually the site of damage caused by the toxin indicates the location of the substrate for that toxin. Terms such as "enterotoxin", "neurotoxin", "leukocidin" or "hemolysin" are sometimes used to indicate the target site of some well-defined protein toxins.
ExotoxinsCertain protein toxins have very specific
cytotoxic activity (i.e., they attack specific cells, for example, tetanus or botulinum toxins), but some (as produced by staphylococci, streptococci, clostridia, etc.) have fairly broad cytotoxic activity and cause nonspecific death of tissues (necrosis). Toxins that are phospholipases may be relatively nonspecific in are phospholipases may be relatively nonspecific in their cytotoxicity because they cleave phospholipids which are components of host cell membranes resulting in the death of the cell by leakage of cellular contents. This is also true of pore-forming "hemolysins" and "leukocidins".
A few protein toxins obviously bring about the death of the host and are known as "lethal toxins“ (e.g., lethal toxin of Bacillus anthracis).
Neutralization of Exotoxins in a Host
Most of the protein toxins are strongly antigenic, because they are foreign substances to the host.
In vivo, specific antibody (antitoxin) neutralizes the toxicity of these bacterial proteins.
Preparation of Toxoid from Exotoxin
Protein toxins are inherently unstable: in time they lose their toxic properties but retain their antigenic ones.
Toxoids are nontoxic preparations which retain their antigenicity and their immunizing capacity. The formation of toxoids can be accelerated by treating formation of toxoids can be accelerated by treating toxins with 0.3-0.4% formalin at 37o C during 4 weeks. The resulting toxoids can be use for artificial immunization against diseases caused by pathogens where the primary determinant of bacterial virulence is toxin production. Toxoids are the immunizing agents against diphtheria and tetanus that are part of the DPT vaccine.
Classification of Exotoxins by Mechanism of Action
1. Toxins which block the protein synthesis on subcellular level have typical A-B structure: fragment B binds to specific host cell receptors and facilitates the entry of fragment A. Function of subunit A can be different:
(a) anti-elongators inactivate ADP-ribose elongation factor-2 (EF-2), thereby leading for inhibition of polypeptide 2 (EF 2), thereby leading for inhibition of polypeptide chain elongation on ribosomes (diphtheria toxin of Corynebacterium diphtheriae and exotoxin A of Pseudomonas aeruginosa);
(b) Shiga toxin of Shigella dysenteriae serotype 1 and enterohemorrhagic strains of Escherichia coli (Stx-toxins) are composed of an enzymatically active A cytotoxic subunit and a B-subunit pentamer that binds to the glycolipid globotriosyl ceramide Gb3 cell receptor. The A-subunit cleaves the 28S rRNA of the 60S ribosomal subunit.
Mechanism of Action of Diphtheria Toxin
Mechanism of Action of Shiga Toxin
Classification of Exotoxins by Mechanism of Action
2. Toxins which damage cytoplasmic membranes.Toxins of this group can disrupt cytoplasmic membrane of
eukaryotic cells by enzymatic hydrolysis or by formation of
pores. Such toxins also improve the spread of microorganisms
in the body of the host (α-toxin of Clostridium perfringens with
activity of phospholipase C). Pore-forming toxins can integrate activity of phospholipase C). Pore-forming toxins can integrate
themselves in the cytoplasmic membrane of target cell, leading
to formation of the canal, which is hydrophilic inside and
hydrophobic outside. The result of the membrane toxin action is
the disruption of the target cell from osmotic shock. Such toxins
increase the permeability of the surface membrane of
erythrocytes (hemolysins=lysins) of Staphylococcus aureus and
Streptococcus pyogenes, causing hemolysis, and leukocytes
(leukocidins) of Staphylococcus aureus and Clostridium
perfringens, causing disruption of leukocytes.
α-toxin of Clostridium perfringens
Classification of Exotoxins by
Mechanism of Action3. Toxins which activate metabolism of secondary
messengers are heat-stable (ST) and heat-labile (LT)
enterotoxins. LT activates cellular adenylate cyclase which
leads to increase of cAMP and elevation of permeability in the
wall of the small intestine. Rapid secretion of electrolytes, with
impairment of sodium chloride absorption and loss of
bicarbonate results in fluid secretion into the small bowel bicarbonate results in fluid secretion into the small bowel
lumen. The process develops into life-threatening massive
diarrhea. Examples of LT producers are Escherichia coli,
Salmonella Typhimurium, Salmonella Enteritidis and
choleragen of Vibrio cholerae. The similar processes occur for
ST through activation of guanylate cyclase for Klebsiella
pneumoniae, Yersinia enterocolitica and Escherichia coli.
Functional blockators are also “murine toxin” of Yersinia pestis
and “edema factor” of Bacillus anthracis. which inactivate
adenylylcyclase by antagonistic enzyme activity.
Mechanism of Action of Cholera Toxin
Classification of Exotoxins by Mechanism of Action
4. Proteases: neurotoxins of Clostridium tetani
and Clostridium botulinum (they are zinc-
metalloendoproteases) which block the release of
glycine and acetylcholine, respectively, and cause glycine and acetylcholine, respectively, and cause
spastic paralysis (tetanus) and flaccid paralysis
(botulism); and lethal factor of Bacillus anthracis
(metalloprotease) which induces the production of
active forms of oxygen into macrophages and
neutrophils and disruption of the cells (cytotoxic
action).
Mechanism of Action of Tetanus Toxin
Botulinum Toxin Mechanism
Classification of Exotoxins by Mechanism of Action
5. Activators of immune response: toxic shock syndrome toxin, enterotoxins and exfoliatins of Staphylococcus aureus and erythrogenins of Streptococcus pyogenes are superantigens. These toxins bind directly to major histocompatibility complex (MHC) class II molecules on antigen-presenting cells and specific Vβ regions of T-cell receptors (TCR), resulting in hyperactivation of up to 20% ofreceptors (TCR), resulting in hyperactivation of up to 20% ofmonocytes/macrophages and T lymphocytes. Activated host cells produce massive amounts of proinflammatory cytokines(IL-1, IL-2, IL-6, TNF-α, IFNs, etc.) and chemokines, activating inflammation and coagulation, causing clinical symptoms that include fever, hypotension, and shock. The major clinical manifestations of the disease are the cumulative actions of the cytokines: they induce shock, high fever and diffuse red rash that later desquamates. Such severe process is called “cytokine storm” and can cause even death of the host.
Mechanism of Action of
Superantigen
Pathogen-Associated Molecular
Patterns (PAMP)Microbial products which stimulate innate
immunity are called pathogen-associated molecular patterns (PAMPs). Pathogen-associated molecular patterns (PAMPs) are on bacteria - they bind to receptors called pattern recognition receptors (PRRs) on host cells (e.g., Toll like receptors). Subsequently on host cells (e.g., Toll-like receptors). Subsequently host innate immune system is activated.
Innate immunity reacts to these common bacterial structures - PAMPs, pathogen-associated molecular patterns. PAMPs are produced only by the bacterial invader and not by the host. If it were the other way around, innate immunity would get "confused" and attack the host instantly and immediately as it is typical of its nature.
PAMPs include
- lipopolysaccharides (LPS) on Gram-negative bacteria- lipoteichoic acid on Gram-positive bacteria
- peptidoglycans- mannan- bacterial RNA and DNA- glucan
Examples of PAMPs
Mechanism of Action of PAMNs
Mechanisms for EscapingHost Defences
• Capsules are typically made of polysaccharides, which are poor immunogens or resemble host tissues (antigenic mimicry). S.pyogenespossesses hyaluronic acid, which is the component of connective tissue, thus, it is not recognized by immune system
• Antigenic variation of periodically changing antigens (borreliae, gonococci, several pathogenic viruses)
Biofilm formation• Biofilm formation
• Formation of L-forms leads to inability of recognition of pathogen as foreign object
• Thick peptidoglycan of G+ m/os and LPS pf G-m/os prevent complement access and damage of cytoplasmic membrane
• Degradation of C5a component of complement limits chemotaxis of S.pyogenes
• Pus formation and the death of neutrophils protects Staph. and Strept. from access of antibodies and antibiotics
• Inactivation of phagocytes
Inactivation of Phagocytes• Enzymes and toxins which lyse phagocytic cells
(O-streptolysin of S.pyogenes and α-toxin of
C.perfringens)
• Capsule and M-protein of S.pyogenes block
intracellular killing
• Blocking of phagolysosome fusion to prevent the
contact with bactericidal contents (Mycobacterium
spp.)
• Plasmocoagulase of S.aureus which ensure clot-
like walling of the site of infection by conversion of
fibrinogen in fibrin
Formation of Biofilm