This review examines Escherichia coli STb enterotoxin, a heat-stable toxin produced by enterotoxigenic E. coli (ETEC) that causes diarrhea in humans and animals. STb is distinguished from other enterotoxins by its resistance to heat and methanol-insolubility. The toxin was identified in 1978 and has been associated primarily with porcine ETEC, though it also occurs occasionally in bovine, chicken, and human strains. Epidemiological studies using DNA probes demonstrate a strong association between STb-producing strains and diarrheal disease in pigs across various ages and serotypes. In humans, STb appears rare and its role in disease remains uncertain, though evidence suggests occasional involvement in traveler's diarrhea. The mature STb protein comprises 48 amino acids containing four cysteine residues that form critical disulfide bonds. The toxin is poorly immunogenic, produced in low amounts by wild-type strains, and regulated by catabolite repression mechanisms. Various in vivo and in vitro assays have been developed to detect STb, revealing that the toxin's activity is not strictly pig-specific but depends on protease inhibition in some animal models. Recent molecular studies show the estB gene is located on transposons, facilitating its horizontal dissemination among ETEC strains.

Key findings

• STb is a heat-stable, 48-amino acid polypeptide with disulfide bonds that is primarily associated with porcine ETEC and strongly linked to diarrhea in pigs of various ages
• The estB gene encoding STb is located on a ~9 kb transposon (Tn4521) that can move between plasmids, enabling spread of this virulence factor among E. coli strains
• STb toxin is not pig-specific; it induces secretory responses in multiple animal species (rats, mice, rabbits, calves, guinea-pigs, chickens) when protease activity is blocked
• STb production is poorly immunogenic and tightly regulated by glucose-mediated catabolite repression, controlled through cAMP-CRP signaling mechanisms
• While STb occasionally appears in human and bovine E. coli isolates, its role in human diarrheal disease remains poorly defined and requires further investigation