The origins and relationships of the pathogenic strains of Vibrio cholerae

Abstract

Sporadic cholera cases had been reported worldwide and that the remnants of the 6th pandemic clone, the 7th pandemic clone and other closely related pathogenic strains are of great importance in their roles to cholera. Sequencing 26 housekeeping genes from V. cholerae isolates that are representatives of the 6th pandemic clone, the 7th pandemic clone, the U.S. Gulf Coast clone, the Australian clone and the pre-7th pandemic strains showed that all of these pathogenic O1 V. cholerae strains evolved from the same lineage, in which during divergence between the 6th pandemic clone and the node for the 7th pandemic clone and the remaining pathogenic O1 El Tor V. cholerae strains, there were eight recombination events and two single mutations. The sequencing of selected seven housekeeping genes (adk, mdh, purM, gyrB, pntA and metE) from 26 V. cholerae isolates (11 pathogenic O1 and 15 non-pathogenic non-O1 non-O139 V. cholerae strains), which consequently led to the generation of seven MLST sequence defined region (416 bp to 591 bp) for each of the seven respective genes, has resulted in a MLST scheme for V. cholerae. This system was able to identify all of the 26 V. cholerae isolates, able to distinguish the O1 from the non-O1 non-O139 V. cholerae strains and was also able to differentiate the pathogenic O1 V. cholerae strains into their five unique groups. The MLST scheme was applied to 19 sporadic strains (one O1 and 18 non-O1 non-O139 V. cholerae strains) and was able to identify 16 of these sporadic strains. The system failed to amplify one, two and five of the selected MLST genes from strains M2551, M2548 and M2550 respectively. Amplification of five virulence-associated genes (ctxA, tcpA, rtxA, acfB and sto/stn) from these 19 sporadic strains by PCR revealed that only one strain M2557 (O1) carries all of these genes, while strains M2548, M2550 and M2551 do not carry any of these genes. The remaining 15 sporadic strains were shown to carry only the rtxA gene.