R-058. Evolution of the Emerging Vibrio cholerae Serogroup O75 and its Relationship to Vibrio cholerae O1 Classical Biotype

C. L. Tarr, M. B. Parsons, M. Miller, K. Greene, C. A. Bopp;
CDC, Atlanta, GA.

Over 200 serogroups of Vibrio cholerae have been identified, but few of these cause the severe watery diarrhea known as cholera. Toxigenic Vibrio cholerae [those producing cholera toxin (CT)] of serogroup O1 have historically been the cause of severe pandemics of cholera. Epidemiological data has documented changes both in the biotype of V. cholerae O1 (from classical to El Tor) and in the emergence of additional toxigenic strains in serogroups O139 and O141. Recently, toxigenic strains of V. cholerae serogroup O75 have been identified as a cause of severe illness in the United States. In addition to the CT gene, these strains also harbor a copy of tcpA, another virulence determinant in V. cholerae that resides on a pathogenicity island called the VPI. Interestingly, based on results of a PCR assay that detects the presence of tcpA alleles, some O75 strains appear to have a tcpA allele that is similar to the V. cholerae O1 classical biotype. The presence of a ‘classical’ allele in an emerging serogroup raises questions about the relationship between strains of serogroup O75 and serogroup O1 classical biotype. Do the O75 strains represent a lineage of epidemic strains that have undergone genetic exchange in the O-antigen biosynthesis region, or are they non-epidemic strains that have acquired some or all of the VPI from strains of the O1 classical biotype? We used nucleotide sequencing of housekeeping genes to examine the evolutionary relationship of an O75 strain to other V. cholerae strains. We also determined the nucleotide sequence of the tcpA gene in the O75 strain and compared it to sequences from other V. cholerae strains. The O75 strain did not fall into the cluster of strains that includes O1 classical biotype strains in trees constructed from housekeeping gene sequences. However, the tcpA sequence from the O75 strain did cluster with the tcpA sequences from classical O1 strains and some O37 strains. Our data are consistent with the hypothesis that the O75 strain is not related to V. cholerae O1 classical biotype but has instead recently acquired some part of the VPI that is closely related to the VPI of V. cholerae O1 classical biotype.