Abstract
Vibrio cholerae in O-group 139 was first isolated in 1992 and by 1993 had been found throughout the Indian subcontinent. This epidemic expansion probably resulted from a single source after a lateral gene transfer (LGT) event that changed the serotype of an epidemic V. cholerae O1 El Tor strain to O139. However, some studies found substantial genetic diversity, perhaps caused by multiple origins. To further explore the relatedness of O139 strains, we analyzed nine sequenced loci from 96 isolates from patients at the Infectious Diseases Hospital, Calcutta, from 1992 to 2000. We found 64 novel alleles distributed among 51 sequence types. LGT events produced three times the number of nucleotide changes compared to mutation. In contrast to the traditional concept of epidemic spread of a homogeneous clone, the establishment of variant alleles generated by LGT during the rapid expansion of a clonal bacterial population may be a paradigm in infections and epidemics.
Highlights
Vibrio cholerae in O-group 139 was first isolated in 1992 and by 1993 had been found throughout the Indian subcontinent
A gmd allele that differed by 113 of the 360 bp sequenced, when compared with sequences in GenBank using BLAST showed greater similarity to gmd from E. coli (AF061251) than gmd from V. cholerae, consistent with lateral gene transfer (LGT) of a homologous gene into the V. cholerae genome
Our results are consistent with clonal expansion and subsequent divergence as described by Spratt and Maiden [26]
Summary
Vibrio cholerae in O-group 139 was first isolated in 1992 and by 1993 had been found throughout the Indian subcontinent This epidemic expansion probably resulted from a single source after a lateral gene transfer (LGT) event that changed the serotype of an epidemic V. cholerae O1 El Tor strain to O139. Vibrio cholerae isolates from this epidemic had a previously unidentified serotype, subsequently designated as O139 Bengal [1,2] This new serotype appears to have resulted when a lateral gene transfer (LGT) event occurred that replaced the 22 kb of the wbf region (encoding the O1 antigen) of a seventh pandemic V. cholerae O1 El Tor strain with a 37-kb region encoding the O139 polysaccharide [3,4,5]. We found putative LGT alleles at all nine loci in the 96 clonally related O139 isolates
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