Molecular epidemiology of O139 Vibrio cholerae: mutation, lateral gene transfer, and founder flush

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.

Figure 1

Bar graph of the number of novel alleles (y-axis) with a specific number of nucleotide differences from the ancestral allele. Two alleles with 24-bp and 113-bp differences are excluded from the graph.

Figure 2

Five groups of related sequence types of Vibrio cholerae O139.