Bacterial Toxins & Genetics

Exotoxins versus endotoxin

Exotoxins. These are polypeptides secreted by both gram-positive and gram-negative bacteria. They are highly toxic in small doses, act on specific targets, are very antigenic and are heat-labile, so they can be inactivated into toxoids and used as vaccines (tetanus, diphtheria). Their genes often sit on plasmids or phages. Many are A-B toxins: the B subunit binds a host receptor and the A subunit carries the enzymatic activity.

Endotoxin. This is lipopolysaccharide (LPS), part of the gram-negative outer membrane, released when the bacterium lyses. Its toxic core is lipid A. Endotoxin is heat-stable and poorly antigenic, cannot be made into a toxoid, and is encoded on the bacterial chromosome.

The staphylococcal enterotoxin that causes food poisoning is an exception to the heat-labile rule: it is preformed and heat-stable, so reheating contaminated food does not prevent the vomiting.

Endotoxin: the lipid A cascade

Cytokines. Lipid A binds CD14 and TLR4 on macrophages, which release IL-1, IL-6 and TNF-α (fever, hypotension) along with nitric oxide, which dilates vessels and deepens shock.

Complement and coagulation. Endotoxin activates complement: C3a causes hypotension and edema, and C5a recruits neutrophils. It also activates Hageman factor (factor XII), starting the coagulation cascade and causing DIC.

"N-DOTS" summarizes endotoxin effects: Nitric oxide, DIC, O2 radicals, TNF, Shock.

Exotoxins by mechanism

EF-2 inactivation. Diphtheria toxin (Corynebacterium diphtheriae, encoded by the beta-prophage) and Pseudomonas exotoxin A both ADP-ribosylate elongation factor 2 (EF-2), halting host protein synthesis. Diphtheria causes a gray pseudomembranous pharyngitis with swollen neck nodes ("bull neck"), and the circulating toxin causes myocarditis and arrhythmias.

60S inactivation. Shiga toxin (Shigella dysenteriae) and Shiga-like toxin (EHEC O157:H7) remove an adenine from the 28S rRNA of the 60S ribosome, halting protein synthesis and enhancing cytokine release. Injured endothelium, especially in the glomeruli, leads to hemolytic uremic syndrome: anemia, thrombocytopenia and acute kidney injury. EHEC does not ferment sorbitol, unlike most E. coli.

cAMP-raising toxins. Cholera toxin ADP-ribosylates Gs and locks it ON. Pertussis toxin ADP-ribosylates Gi and disables the OFF signal, so cAMP also rises; the disease is whooping cough with marked lymphocytosis. The heat-labile toxin (LT) of enterotoxigenic E. coli raises cAMP the same way, and anthrax edema factor is itself an adenylate cyclase.

Cholera turns the "on" switch up; pertussis takes the "off" switch away. Both leave cAMP high.

SNARE proteases. Tetanospasmin and botulinum toxin are proteases that cleave SNARE proteins, so synaptic vesicles cannot fuse and neurotransmitter is not released. Tetanospasmin (Clostridium tetani, from wound spores) travels retrogradely to the spinal cord and blocks release of inhibitory GABA and glycine from Renshaw cells, producing spastic paralysis, lockjaw and opisthotonus. Botulinum toxin blocks acetylcholine release at the neuromuscular junction, producing flaccid paralysis. Infants who swallow spores in honey become "floppy babies"; adults usually ingest preformed toxin in home-canned food.

Superantigens. TSST-1 (S. aureus) and streptococcal pyrogenic exotoxin A (S. pyogenes) cross-link MHC II to the T-cell receptor OUTSIDE the antigen groove. Many T cells activate nonspecifically at once and release IL-2, IFN-γ and TNF, causing toxic shock with fever, rash, desquamation and hypotension. Staphylococcal toxic shock is menstrual (tampons) or surgical and often has negative blood cultures.

Membrane-damaging toxins. Other exotoxins attack membranes directly. Clostridium perfringens alpha toxin is a phospholipase (lecithinase) that causes myonecrosis in gas gangrene. Streptolysin O is a pore-forming hemolysin, and antibodies to it (ASO) mark recent streptococcal infection.

Phage-encoded toxins and gene transfer

Lysogenic conversion. Several key exotoxins are made only when the bacterium carries a lysogenic phage whose genome encodes the toxin. The mnemonic is ABCDE: group A strep erythrogenic toxin, Botulinum toxin (some strains), Cholera toxin, Diphtheria toxin and shiga-like toxin of Enterohemorrhagic E. coli. TSST-1 is not on the list.

Transformation. Naturally competent bacteria take up naked DNA from the environment: "SHiN," for S. pneumoniae, H. influenzae and Neisseria. Adding DNase blocks it. Griffith showed that heat-killed encapsulated pneumococci could transform live unencapsulated ones, and Avery used DNase to prove that DNA, not protein, is the transforming principle.

Transduction. A bacteriophage carries bacterial genes to a new host. In generalized transduction (lytic cycle), random host DNA is mis-packaged into phage heads. In specialized transduction (lysogenic cycle), a prophage excises imprecisely and takes the genes next to its insertion site. Toxin genes travel with lysogenic phages, which is why the ABCDE toxins are taught with specialized transduction.

Conjugation. DNA moves from cell to cell through a sex pilus. In F+ × F⁻ mating the F plasmid transfers and the recipient becomes F+. Hfr cells have F integrated into the chromosome and can transfer chromosomal genes. Plasmids carry many antibiotic-resistance genes.

Transposition. Transposons ("jumping genes") move between plasmid and chromosome. Carried on plasmids, they spread antibiotic-resistance genes and create new plasmid combinations.