Figure 1. Steps in the evolution of the seventh pandemic Vibrio cholerae.

Environmental V. cholerae indigenous in coastal waters can harbor genomic islands (GIs) by horizontal gene transfer, rendering it pathogenic. Pathogenesis of toxigenic (toxin-producing) V. cholerae critically depends on the production of the cholera toxin, which is responsible for the cholera symptoms, and the toxin-coregulated pilus (TCP). The genes for the cholera toxin (ctx) are from the filamentous bacteriophage, CTXφ, that has been incorporated into the genome. The genes in the TCP island encode factors necessary for the colonization of the small intestine in the human host after ingestion of contaminated water. Additionally, seventh pandemic strains are distinguishable from pre-seventh pandemic strains due to the acquisition of additional GIs, the Vibrio seventh pandemic (VSP) islands.