Bacterial Toxins, Genetics & Virulence
A Step 1 high-yield micro lesson on bacterial toxins, genetics, and virulence — organizing exotoxin mechanisms (EF-2 ADP-ribosylation, ↑cAMP secretory, 60S/Shiga, SNARE proteases, superantigens), the exotoxin-vs-endotoxin contrast, gene transfer (transformation/transduction/conjugation, with lysogenic conversion carefully distinguished from specialized transduction), and non-toxin virulence factors, all tied to vignette buzzwords and current treatments. Includes verified Commons images and genuine classic mnemonics (ABCD'S, SHiN, the EF-2 pair).
Why toxins & genetics own this exam block
Boards love bacterial virulence because a single toxin often is the disease — knock out the toxin and the organism is harmless. Two big ideas unlock the whole topic.
First, exotoxin vs endotoxin. Exotoxins are secreted polypeptides made by both Gram-positive and Gram-negative bacteria; they are highly potent, act on specific targets, and can be inactivated into toxoids for vaccines (tetanus, diphtheria). Endotoxin is lipopolysaccharide (LPS) — its lipid A component is embedded in the Gram-negative outer membrane, is released on lysis, and drives a nonspecific inflammatory cascade (fever, hypotension, DIC) via TLR4/CD14.
Second, where toxin genes come from. Many of the highest-yield toxins are not chromosomal — they ride in on mobile genetic elements (lysogenic bacteriophage or plasmids). That single fact ties toxins to bacterial genetics and explains why a harmless strain can suddenly become lethal after phage infection (lysogenic conversion).
Exotoxin vs Endotoxin — the classic 2-column
| Feature | Exotoxin | Endotoxin |
|---|---|---|
| Source | Certain Gram(+) and Gram(−) bacteria; secreted | Outer membrane of Gram(−) bacteria; released on lysis |
| Chemistry | Polypeptide (often A-B subunit) | Lipopolysaccharide (lipid A = toxic part) |
| Gene location | Often plasmid or bacteriophage | Bacterial chromosome |
| Potency | Very high (fatal in µg) | Low (needs large dose) |
| Clinical effect | Specific to the toxin | Fever, hypotension, DIC/shock (nonspecific) |
| Mediators | Varies by target | IL-1, IL-6, TNF-α via TLR4/CD14 |
| Antigenicity / toxoid | Highly antigenic; toxoid vaccines exist | Poorly antigenic; no toxoid |
| Heat stability | Destroyed at 60 °C (except staph enterotoxin & E. coli heat-stable ST) | Stable at 100 °C |
| Prototype diseases | Tetanus, botulism, diphtheria, cholera | Meningococcemia, Gram(−) sepsis |
- ADP-ribosylation of EF-2 → protein-synthesis arrest: Diphtheria toxin (C. diphtheriae) and Exotoxin A (P. aeruginosa) both inactivate elongation factor-2.
- ↑ cAMP → secretory watery diarrhea: Cholera toxin permanently activates Gs; E. coli LT does the same; Pertussis toxin inactivates Gi; anthrax edema factor is itself an adenylate cyclase. (E. coli ST instead ↑ cGMP.)
- 60S ribosome inactivation: Shiga & Shiga-like toxins remove an adenine from the 28S rRNA of the 60S subunit → halt translation and damage endothelium → HUS.
- SNARE protease → blocked neurotransmission: Tetanospasmin blocks release of inhibitory GABA/glycine (spinal Renshaw cells/interneurons) → spastic paralysis; Botulinum toxin blocks ACh at the NMJ → flaccid paralysis. Both cleave SNARE proteins.
- Superantigens → cytokine storm: TSST-1 (S. aureus) and SpeA (S. pyogenes) bridge MHC II ↔ TCR (Vβ) outside the peptide groove → polyclonal T-cell activation → flood of IL-2, IFN-γ, TNF-α → shock.
- (Bonus — cytolytic) C. perfringens α-toxin is a lecithinase (phospholipase C) → myonecrosis; streptolysin O lyses RBCs and is the target of the ASO titer.
Master toxin table (mechanism → effect)
| Toxin | Organism | Mechanism / target | Result |
|---|---|---|---|
| Diphtheria toxin | C. diphtheriae | ADP-ribosylates EF-2 | Pseudomembranous pharyngitis, myocarditis, arrhythmia |
| Exotoxin A | P. aeruginosa | ADP-ribosylates EF-2 | Host-cell death (pneumonia, sepsis) |
| Cholera toxin | V. cholerae | Activates Gs → ↑cAMP | Rice-water diarrhea |
| LT / ST | ETEC | LT ↑cAMP; ST ↑cGMP | Watery traveler's diarrhea |
| Pertussis toxin | B. pertussis | Inhibits Gi → ↑cAMP | Whooping cough, lymphocytosis |
| Shiga / Shiga-like | Shigella / EHEC O157:H7 | Inactivates 60S (cleaves 28S rRNA) | Bloody diarrhea, HUS |
| Tetanospasmin | C. tetani | SNARE protease; blocks GABA/glycine | Spastic paralysis, lockjaw |
| Botulinum toxin | C. botulinum | SNARE protease; blocks ACh | Flaccid paralysis |
| α-toxin | C. perfringens | Lecithinase (phospholipase C) | Gas gangrene, myonecrosis |
| TSST-1 | S. aureus | Superantigen | Toxic shock, desquamating rash |
| Erythrogenic/SpeA | S. pyogenes | Superantigen | Scarlet fever, strep TSS |
| Anthrax toxin (PA/EF/LF) | B. anthracis | EF = adenylate cyclase; LF = protease | Edema + cell death |
- "Gray, adherent pseudomembrane that bleeds when scraped" + bull neck + unvaccinated → *Corynebacterium diphtheriae* → diphtheria antitoxin FIRST, then penicillin/erythromycin; immunize with toxoid.
- Child, undercooked hamburger → bloody diarrhea, now pale with ↓platelets, ↑creatinine, schistocytes → EHEC O157:H7 (Shiga-like toxin → HUS) → supportive/dialysis; do NOT give antibiotics (raises HUS risk).
- Floppy infant, poor feeding, constipation, fed honey, descending weakness → *Clostridium botulinum* → human botulism immune globulin (BIG-IV / "BabyBIG") + supportive care.
- Deep puncture wound → lockjaw, risus sardonicus, opisthotonos → *Clostridium tetani* → tetanus immune globulin (TIG) + metronidazole + wound debridement + vaccination.
- Menstruating woman, tampon use: fever, hypotension, diffuse macular erythroderma, desquamating palms/soles → S. aureus TSST-1 → remove tampon, fluids/pressors, antistaphylococcal agent (vancomycin ± clindamycin to suppress toxin).
- Abrupt voluminous painless "rice-water" stools, severe dehydration in a traveler → *Vibrio cholerae* → aggressive oral/IV rehydration ± doxycycline.
- Paroxysmal cough with inspiratory "whoop," post-tussive emesis, marked lymphocytosis, unvaccinated → *Bordetella pertussis* → macrolide (azithromycin).
- ABCD'S — toxins encoded by a lysogenic bacteriophage (lysogenic conversion): group A strep erythrogenic toxin, Botulinum, Cholera, Diphtheria, Shiga.
- "Two bugs ADP-ribosylate EF-2": Diphtheria toxin + Pseudomonas exotoxin A → protein synthesis stops.
- The cAMP-raising toxins (group them): Cholera (activates Gs), Pertussis (inhibits Gi), E. coli LT, and anthrax edema factor all ↑cAMP; E. coli ST instead ↑cGMP.
- Superantigens bridge MHC II ↔ TCR: TSST-1 (staph) + SpeA (strep) → IL-2, IFN-γ, TNF-α storm.
- SHiN — the naturally transformable bugs that also make IgA protease and are encapsulated: *S. pneumoniae, H. influenzae, Neisseria*.
- Paralysis directions: teta-N-us = No relaxation (spastic); botulism = flaccid/floppy.
Bacterial genetics: how virulence spreads
Three mechanisms move genes between bacteria — and they are exactly how toxin and antibiotic-resistance genes propagate.
1. Transformation — uptake of naked DNA from the environment by naturally competent bacteria (SHiN: S. pneumoniae, H. influenzae, Neisseria). Adding DNase to the medium abolishes it, proving free DNA is the vehicle.
2. Transduction — DNA moved inside a bacteriophage. In generalized transduction a lytic phage accidentally packages random host DNA during assembly. In specialized transduction a lysogenic prophage excises imprecisely and drags an adjacent host chromosomal gene into the next cell. Do not confuse this with lysogenic conversion, where the prophage's own genes give the host a new trait — that (not specialized transduction) is how the ABCD'S toxin genes are carried and switched on.
3. Conjugation — direct transfer through a sex pilus. F⁺ × F⁻ transfers the plasmid only (recipient becomes F⁺); an Hfr cell (F factor integrated into the chromosome) can transfer chromosomal genes. Conjugative R (resistance) plasmids are the main highway for spreading multidrug resistance.
Transposition ("jumping genes") then shuffles resistance/virulence cassettes between plasmid and chromosome, assembling multidrug-resistant strains.
- Capsule (polysaccharide): antiphagocytic — the shared weapon of the encapsulated bugs; basis of vaccines, conjugated to protein to elicit T-cell help in children < 2 yr.
- IgA protease: cleaves secretory IgA so the organism can colonize mucosa — made by SHiN (S. pneumoniae, H. influenzae, Neisseria).
- Protein A (S. aureus): binds the Fc region of IgG → blocks opsonization and complement.
- M protein (S. pyogenes): antiphagocytic; molecular mimicry with cardiac myosin → rheumatic fever.
- Biofilms: S. epidermidis on catheters/prosthetics; P. aeruginosa in CF airways — shield from antibiotics and immune cells.
- Pili/fimbriae: adhesion to host cells (and the conjugation apparatus).
Organism → key lab feature → disease → treatment
| Organism | Identifying feature / lab clue | Disease | Treatment |
|---|---|---|---|
| C. diphtheriae | Gram(+) rod, metachromatic granules, tellurite/Löffler agar | Diphtheria (pseudomembrane, myocarditis) | Antitoxin + penicillin/erythromycin |
| C. tetani | Anaerobe, "tennis-racket" spores | Tetanus (spastic paralysis) | TIG + metronidazole + debridement |
| C. botulinum | Anaerobe; toxin in canned food/honey | Botulism (flaccid paralysis) | Antitoxin (adult) / BIG-IV (infant) |
| C. perfringens | Double zone of hemolysis, gas in tissue | Gas gangrene; food poisoning | Debridement + penicillin + clindamycin |
| V. cholerae | Comma-shaped, oxidase(+), alkaline TCBS | Cholera (rice-water stool) | Rehydration ± doxycycline |
| B. pertussis | Gram(−) coccobacillus, Bordet-Gengou agar | Whooping cough | Azithromycin |
| S. aureus (TSST-1) | Gram(+) cocci clusters, coagulase(+) | Toxic shock syndrome | Remove source + vancomycin ± clindamycin |
| S. pyogenes | Gram(+) cocci chains, β-hemolytic, bacitracin-sensitive, ↑ASO | Scarlet fever / strep TSS | Penicillin (+ clindamycin in TSS) |
| EHEC O157:H7 | Sorbitol-negative on MacConkey; no invasion | Bloody diarrhea → HUS | Supportive; avoid antibiotics |
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