THE 6TH AND 7TH CHOLERA PANDEMICS ARE DUE TO INDEPENDENT CLONES SEPARATELY DERIVED FROM ENVIRONMENTAL, NONTOXIGENIC, NON-O1 VIBRIO-CHOLERAE

The DNA sequences of the asd genes from 45 isolates of Vibrio cholerae (19 clinical O1 isolates, 2 environmental nontoxigenic O1 isolates, a nd 23 isolates with different non-O1 antigens) were determined. No dif ferences were found within either sixth- or seventh-pandemic isolates; however, variation was found between the two forms and among the non- O1 isolates. O139 isolates had sequences identical to those of seventh -pandemic isolates. Phylogenetic trees with Vibrio mimicus as the outg roup suggest that the sixth-pandemic, seventh-pandemic, and U.S. Gulf isolates are three clones that have evolved independently from differe nt Lineages of environmental, nontoxigenic, non-O1 V. cholerae isolate s. There is evidence for horizontal transfer of O antigen, since isola tes with nearly identical asd sequences had different O antigens, and isolates with the O1 antigen did not cluster together but were found i n different lineages. We also found evidence for recombination events within the asd gene of V. cholerae. V. cholerae may have a higher leve l of genetic exchange and a lower level of clonality than species such as Salmonella enterica and Escherichia coli.