Genetic diversity and population structure of Vibrio cholerae

Multilocus enzyme electrophoresis (MLEE) of 397 Vibrio cholerae isolates, i ncluding 143 serogroup reference strains and 244 strains from Mexico and Gu atemala, identified 279 electrophoretic types (ETs) distributed in two majo r divisions (I and II). Linkage disequilibrium was demonstrated in both div isions and in subdivision Ic of division I but not in subdivision Ia, which includes 76% of the ETs, Despite this evidence of relatively frequent reco mbination, clonal lineages may persist for periods of time measured in at l east decades. In addition to the pandemic clones of serogroups O1 and O139, which form a tight cluster of four ETs in subdivision Ia, MLEE analysis id entified numerous apparent clonal lineages of non-O1 strains with intercont inental distributions. A clone of serogroup O37 that demonstrated epidemic potential in the 1960s is closely related to the pandemic O1/O139 clones, b ut the nontoxigenic O1 Inaba El Tor reference strain is not. A strain of se rogroup O22, which has been identified as the most likely donor of exogenou s rfb region DNA to the O1 progenitor of the O139 clone, is distantly relat ed to the O1/O139 clones. The close evolutionary relationships of the O1, O 139, and O37 epidemic clones indicates that new cholera clones are likely t o arise by the modification of a lineage that is already epidemic or is clo sely related to such a clone.