Microbiology chapters
Pathogenicity, colonisation, immune evasion and toxins
How bacteria cause disease: transmission and portals of entry, adhesion and colonisation, invasion, ways of escaping immune defences (capsules, IgA protease, antigenic variation, intracellular survival), and the toxins: exotoxins with their mechanisms versus endotoxin (lipopolysaccharide), with the classic exam examples.
- Pathogenicity is the ability to cause disease; virulence is the degree (measured as ID50/LD50); bacteria cause disease by adhering (pili, fimbriae, biofilms), invading, evading immunity (polysaccharide capsules block phagocytosis, protein A binds IgG Fc, IgA protease, antigenic variation of pili in Neisseria, survival inside macrophages) and producing toxins.
- Exotoxins are proteins secreted mainly by Gram-positive (and some Gram-negative) bacteria; they are highly potent, specific, heat-labile (mostly), strongly antigenic and can be made into toxoids (tetanus, diphtheria); many are A-B toxins: diphtheria and Pseudomonas exotoxin A ADP-ribosylate EF-2, cholera and E. coli LT activate Gs (↑cAMP), pertussis inactivates Gi, tetanus blocks inhibitory (glycine, GABA) release, botulinum blocks acetylcholine release.
- Endotoxin is the lipopolysaccharide (lipid A) forming part of the Gram-negative outer membrane: released when bacteria die, heat-stable, weakly antigenic, no toxoid, and triggers TNF, IL-1 → fever, hypotension (septic shock) and DIC.
On this page
Basic terms
- Pathogen: an organism that can cause disease. Opportunistic pathogens cause disease mainly in people with weakened defences (Pseudomonas, Candida, Pneumocystis).
- Pathogenicity: the ability to cause disease. Virulence: the degree of pathogenicity, measured by the ID50 (dose infecting 50% of hosts) or LD50 (dose killing 50%). A low ID50 means high infectivity: Shigella needs only a few organisms, while Vibrio cholerae and Salmonella need large numbers (stomach acid kills many).
- Normal flora: harmless resident organisms that protect by competing with pathogens (colonisation resistance), making vitamin K, and stimulating immunity. Antibiotics that disturb them allow C. difficile or Candida to overgrow.
- Koch's postulates: the organism is found in every case, can be grown in pure culture, reproduces the disease when inoculated, and is recovered again. Exceptions: M. leprae and T. pallidum cannot be grown in artificial media.
Stages of infection
1. Transmission and portals of entry
Respiratory droplets and aerosols (TB, diphtheria, pertussis), faecal-oral (typhoid, cholera), sexual (gonorrhoea, syphilis), blood and needles (hepatitis B), vectors (plague, rickettsiae), direct contact and wounds (staphylococci, tetanus), vertical (mother to baby: syphilis, Listeria, group B streptococcus).
2. Adherence and colonisation
- Pili (fimbriae): hair-like structures that attach to host cells (E. coli in UTIs, N. gonorrhoeae).
- Adhesins: lipoteichoic acid and M protein (streptococci), filamentous haemagglutinin (Bordetella).
- Biofilms: bacteria in a slime matrix stuck to surfaces such as catheters, prosthetic valves and joints (S. epidermidis, Pseudomonas); they resist antibiotics and host defences. Dental plaque is a biofilm (Streptococcus mutans).
3. Invasion and spread
Enzymes that help spread: hyaluronidase ("spreading factor"), collagenase, streptokinase (dissolves clots), DNase, coagulase (S. aureus: forms a fibrin wall around the lesion), lecithinase (C. perfringens α-toxin).
4. Evading host defences
| Strategy | Examples |
|---|---|
| Polysaccharide capsule (antiphagocytic; the most important virulence factor of many organisms) | S. pneumoniae, H. influenzae type b, N. meningitidis, Klebsiella, Salmonella Typhi (Vi antigen), Cryptococcus (fungus). Capsule antibodies (from vaccines) give protection |
| Protein A | Staphylococcus aureus: binds the Fc part of IgG, blocking opsonisation |
| M protein | Streptococcus pyogenes: blocks complement and phagocytosis; antibodies cross-react with heart (rheumatic fever) |
| IgA protease | N. gonorrhoeae, N. meningitidis, H. influenzae, S. pneumoniae: cleave mucosal IgA |
| Antigenic variation | N. gonorrhoeae (pili and Opa proteins: no lasting immunity, repeated infections), Borrelia recurrentis (relapsing fever), trypanosomes; among viruses, influenza (drift and shift) and HIV |
| Intracellular survival | M. tuberculosis, M. leprae, Salmonella, Listeria, Legionella, Brucella, Francisella, Yersinia: survive inside macrophages (block phagosome-lysosome fusion, escape into cytoplasm) → controlled by cell-mediated immunity and granulomas |
| Catalase and superoxide dismutase | Protect against phagocyte killing; catalase-positive organisms (S. aureus, Aspergillus, Nocardia, Serratia, Burkholderia) infect people with chronic granulomatous disease |
| Leukocidins | S. aureus Panton-Valentine leukocidin kills white cells |
Encapsulated organisms (pneumococcus, Hib, meningococcus) cause overwhelming sepsis after splenectomy or in sickle cell disease (autosplenectomy); give vaccines and prophylaxis.
Toxins
Exotoxins vs endotoxin
| Feature | Exotoxin | Endotoxin |
|---|---|---|
| Source | Secreted by living bacteria; Gram-positive and Gram-negative | Outer membrane of Gram-negative bacteria; released on death or lysis |
| Chemistry | Protein (polypeptide) | Lipopolysaccharide: toxic part is lipid A |
| Genes | Often on plasmids or bacteriophages | Bacterial chromosome |
| Toxicity | Very high (tiny doses fatal: botulinum toxin is the most potent known) | Lower |
| Specificity | Specific actions on particular cells | General effects (fever, shock) |
| Heat | Most heat-labile (destroyed at 60–100 °C; S. aureus enterotoxin and E. coli ST are exceptions) | Heat-stable (survives autoclaving) |
| Antigenicity | Strong; neutralised by antitoxin | Weak |
| Toxoid | Yes (vaccines: tetanus, diphtheria) | No |
| Fever | Usually no | Yes (pyrogenic) |
Endotoxin (LPS)
- Structure: lipid A (toxic, in the membrane), core polysaccharide, O antigen (variable, used to type organisms such as E. coli O157).
- Acts through TLR4 on macrophages → TNF, IL-1, IL-6, nitric oxide → fever, vasodilation and hypotension (septic shock), activation of complement and coagulation (DIC), neutrophilia.
- Also activates factor XII (Hageman factor) → kinins.
- Detected by the Limulus amoebocyte lysate test (used to check IV fluids for pyrogens).
- Gram-negative septicaemia: E. coli, Klebsiella, Pseudomonas, N. meningitidis (Waterhouse-Friderichsen syndrome).
Important exotoxins
| Toxin | Organism | Mechanism | Effect |
|---|---|---|---|
| Diphtheria toxin | Corynebacterium diphtheriae (gene carried by β-phage) | ADP-ribosylates EF-2 → stops protein synthesis | Pseudomembrane, myocarditis, neuropathy |
| Exotoxin A | Pseudomonas aeruginosa | ADP-ribosylates EF-2 | Tissue damage |
| Cholera toxin | Vibrio cholerae | ADP-ribosylates Gs → adenylyl cyclase permanently on → ↑ cAMP → Cl⁻ and water secretion | Rice-water diarrhoea |
| Heat-labile toxin (LT) | ETEC | Same as cholera toxin (↑ cAMP) | Travellers' diarrhoea |
| Heat-stable toxin (ST) | ETEC | Activates guanylyl cyclase → ↑ cGMP | Watery diarrhoea |
| Pertussis toxin | Bordetella pertussis | ADP-ribosylates Gi (inactivates it) → ↑ cAMP | Lymphocytosis, whooping cough, insulin release |
| Shiga toxin / Shiga-like (verotoxin) | Shigella dysenteriae, EHEC O157:H7 | Cleaves 60S rRNA → stops protein synthesis | Bloody diarrhoea, haemolytic uraemic syndrome |
| Tetanus toxin (tetanospasmin) | Clostridium tetani | Travels up nerves to the spinal cord; cleaves synaptobrevin in inhibitory interneurons → blocks glycine and GABA release | Spastic paralysis, trismus (lockjaw), risus sardonicus, opisthotonos |
| Botulinum toxin | Clostridium botulinum | Cleaves SNARE proteins at the neuromuscular junction → blocks acetylcholine release | Descending flaccid paralysis, diplopia, dysphagia, respiratory failure; floppy baby (infant botulism from honey) |
| α-toxin (lecithinase, phospholipase C) | Clostridium perfringens | Destroys cell membranes | Gas gangrene, haemolysis |
| Toxin A and B | Clostridioides difficile | Damage actin cytoskeleton of enterocytes | Pseudomembranous colitis |
| Anthrax toxin | Bacillus anthracis | Oedema factor (an adenylyl cyclase → ↑ cAMP) + lethal factor | Black eschar, oedema |
| Streptolysin O | S. pyogenes | Pore-forming haemolysin; antigenic (ASO titre) | β-haemolysis; ASO rises after infection |
| Superantigens: TSST-1, staphylococcal enterotoxins, streptococcal pyrogenic exotoxin (erythrogenic toxin) | S. aureus, S. pyogenes | Bind MHC class II and T-cell receptor outside the antigen groove → massive T-cell activation and cytokine storm (IL-2, IFN-γ, TNF) | Toxic shock syndrome (fever, rash, hypotension, desquamation), scarlet fever rash; enterotoxins cause food poisoning (vomiting within 1–6 hours) |
Tetanus = toxin blocks inhibitory neurons → spasm. Botulism = toxin blocks ACh release → flaccid paralysis.
Staphylococcal food poisoning (preformed, heat-stable enterotoxin in custard, milk products, meat) and Bacillus cereus (reheated rice) cause vomiting within hours; reheating the food does not destroy the staphylococcal toxin.
Toxoids and antitoxins
- Toxoid: exotoxin treated with formaldehyde → loses toxicity but keeps antigenicity → active immunity (DPT/pentavalent vaccine: diphtheria and tetanus toxoids).
- Antitoxin: ready-made antibody → passive immunity (tetanus immunoglobulin, diphtheria antitoxin, botulinum antitoxin).
Expect: pathogenicity vs virulence and ID50, Koch's postulates and exceptions, adhesins and biofilms, spreading enzymes, capsules and asplenia, protein A, IgA protease, antigenic variation (gonococcus), intracellular organisms, exotoxin vs endotoxin table, lipid A and septic shock, mechanisms of diphtheria, cholera, pertussis, Shiga, tetanus and botulinum toxins, superantigens and toxic shock, staphylococcal food poisoning, toxoids vs antitoxins.
Quick revision
- Virulence measured by ID50/LD50; Shigella has a very low ID50.
- Capsule: antiphagocytic; asplenia → pneumococcus, Hib, meningococcus.
- Protein A binds IgG Fc; IgA protease in Neisseria, Haemophilus, pneumococcus.
- Antigenic variation: gonococcal pili, Borrelia recurrentis.
- Exotoxin: protein, heat-labile, toxoid. Endotoxin: LPS lipid A, Gram-negative, heat-stable, septic shock, DIC.
- Diphtheria: EF-2. Cholera: Gs ↑cAMP. Pertussis: Gi off. Shiga: 60S.
- Tetanus: spastic (glycine/GABA). Botulinum: flaccid (ACh).
- Superantigens: TSST-1, scarlet fever toxin.
Written by The House Job from standard microbiology references (Levinson's Review of Medical Microbiology and Immunology).
FAQ
How does endotoxin differ from exotoxin?
Endotoxin is the lipopolysaccharide (lipid A) of the Gram-negative outer membrane, released when bacteria die; it is heat-stable, weakly antigenic and cannot be made into a toxoid. Exotoxins are secreted proteins, very potent, mostly heat-labile and can be made into toxoids.
How does cholera toxin cause diarrhoea?
It ADP-ribosylates the Gs protein, keeping adenylyl cyclase switched on; cAMP rises and gut cells pour out chloride and water.
Why do people without a spleen get severe infections with encapsulated bacteria?
The spleen removes encapsulated bacteria and makes antibodies against capsule polysaccharides; without it, organisms such as pneumococcus spread rapidly in the blood.
How do tetanus and botulinum toxins differ?
Tetanus toxin blocks inhibitory neurotransmitter release in the spinal cord, causing spastic paralysis; botulinum toxin blocks acetylcholine release at the neuromuscular junction, causing flaccid paralysis.