ctxAB

ctxAB refers to the genes that encode the cholera toxin subunits A and B in Vibrio cholerae. The genes are carried by the CTXφ filamentous bacteriophage and are integrated into the chromosome of toxigenic strains, enabling production of the classic enterotoxin.

Cholera toxin is an AB5 toxin composed of one enzymatically active A subunit (CtxA) and a pentamer of B subunits (CtxB). The B subunits assemble into a pentamer that binds to the GM1 ganglioside receptor on intestinal epithelial cells, enabling uptake of the A subunit. After internalization and proteolytic processing, the A1 fragment ADP-ribosylates the Gs alpha subunit, persistently activating adenylate cyclase and raising intracellular cAMP. The cAMP surge drives chloride and water secretion into the intestinal lumen, causing the profuse watery diarrhea characteristic of cholera.

Expression of ctxAB is controlled by the ToxR regulon; ToxR/ToxS activate ToxT, which directly induces ctxAB and tcpA transcription in response to environmental cues such as bile, temperature, and osmolarity. The CTX phage provides the ctxAB genes and is dependent on the toxin-coregulated pilus (TCP) for infection; in lysogenic strains, ctxAB expression can be induced in the host environment.

Clinical and public health significance: the presence of ctxAB marks toxigenic V. cholerae; cholera toxin is the primary virulence factor causing the secretory diarrhea of cholera. Detection of ctxAB is used in typing and diagnostic assays.