The sixth and seventh cholera pandemics are due to independent clones separately derived from environmental, nontoxigenic, non-O1 Vibrio cholerae

DNA sequence analysis of the asd genes from 45 isolates of Vibrio cholerae revealed variation between the two forms of environmental nontoxigenic isolates and among 19 clinical O1 isolates, but no differences in the sixth- and seventh-pandemic isolates. Phylogenetic trees of Vibrio cholerae with V. mimicus as the outgroup showed that sixth- and seventh-pandemic and US Gold isolates evolved from the different lineages of environmental nontoxigenic O1 isolates.