Cholera toxin (CTX) genetic element in Vibrio cholerae O139

PFGE analysis of the NotI- and SfiI-digested genome of Vibrio cholerae O139 strains isolated from different epidemic regions of India showed that all the strains are of clonal origin and the genome size is about 2.2 Mb. An analysis of the electrophoretic profiles of the genome of O139 strains, the RFLP of the cholera toxin (ctx) gene and Southern blot hybridization of NotI-digested genomes of classical, El Tor and O139 with a NotI-linking clone of classical strain 569B, suggest that these strains closely resemble V. cholerae O1 biotype El Tor, but are widely different from the classical O1 vibrios. Using restriction enzymes which cleave a single site in either the core region or in the direct repeat sequence (RS) of the CTX genetic element, it has been shown that the genome of most of the O139 strains has two copies of the ctx gene in tandem connected by two RSs. The chromosomal location of the CTX genetic element in the O139 strain is the same as that reported for El Tor vibrios. The organization of the virulence gene cassettes in different O139 strains shows genetic heterogeneity in the population. Whilst most of the epidemic strains have two copies of the CTX genetic element, in some strains the number of elements has been amplified and in at least one strain a single copy of the element has been deleted.