A study on the evolution of atypical Vibrio cholerae O1 El Tor variants

Abstract

Vibrio cholerae O1 serogroup strains have been categorized into two biotypes – classical and El Tor. The pathogenesis of cholera is related mainly to the production of cholera toxin, encoded by ctxAB, which is located in the CTX prophage integrated on the V. cholerae chromosome. V. cholerae O1 El Tor biotype strains usually used to produce the biotype-specific cholera toxin, classical biotype strains contain a biotype-specific CTX phage, CTXcla. The seventh cholera pandemic which began in 1961 has been caused by the El Tor biotype strains. Recently, the prototype El Tor strains, or Wave 1 strains that produce biotype-specific cholera toxin are being replaced by atypical El Tor variants that harbor classical cholera toxin. Wave 2 strains contain a tandem repeat of CTX-2, which harbors ctxB1, on chromosome 2 and various arrays, such as no element, RS1:RS1, TLC:RS1:RS1, and TLC:RS1:CTX-1:RS1, on chromosome 1. Wave 3 strains are El Tor biotype strains that harbor any of CTX-3, 4, 5, 6, CTX-3b, or CTX-6b on chromosome 1 and lack elements on chromosome 2. A phylogenetic analysis based on SNPs throughout the V. cholerae genome indicated that the typical and atypical El Tor strains originated from a common ancestor in the 1950’s, which suggest that atypical strains arose from the prototype strains by replacing the CTX phages. This study examined the molecular mechanisms that generated Wave 2 strains and Wave 3 strains from Wave 1 strains. V. cholerae O1 serotype Ogawa strain, V212-1, was isolated in 1991 in India. The precise CTX array of V212-1 determined that in this study is TLC:RS1:CTX-1:RS1 on chromosome 1 and CTX-2:CTX-2 on chromosome 2. V212-1 considered intermediary strain because contains CTX-1 (Wave 1) on chromosome 1 and CTX-2 (Wave 2) on chromosome 2. Wave 2 and Wave 3 strains may have been generated from this intermediary strain. This study demonstrated that Wave 2 strains can be generated directly from V212-1 by progressive elimination of CTX-1 and RS1 through intra-strand recombination and the intermediary strain, V212-1 might be the ancestor Wave 2 strains. New types of the CTX phage could be generated from the V212-1 by inter-chromosomal recombination between 2 different CTX phages and recombination between the CTX phage and RS1. These new CTX phages can be transduced into other El Tor strains, transforming them into Wave 3 atypical strains. This study demonstrated instance of how a single-segment-genome CTX phage re-organizes its genome through recombination between different types of phage, leading to generation of new phage variants and atypical El Tor strains and the intra-strand and inter-strand recombination between CTX phage and RS1 element played an important role in the evolution of CTX phage and V. cholerae