Vibrio cholerae classical biotype strains reveal distinct signatures in Mexico

Abstract

Vibrio cholerae O1 classical (CL) biotype caused the fifth and sixth pandemics, and probably the earlier cholera pandemics, before the El Tor (ET) biotype initiated the seventh pandemic in Asia in the 1970s by completely displacing the CL biotype. Although the CL biotype was thought to be extinct in Asia and although it had never been reported from Latin America, V. cholerae CL and ET biotypes, including a hybrid ET, were found associated with areas of cholera endemicity in Mexico between 1991 and 1997. In this study, CL biotype strains isolated from areas of cholera endemicity in Mexico between 1983 and 1997 were characterized in terms of major phenotypic and genetic traits and compared with CL biotype strains isolated in Bangladesh between 1962 and 1989. According to sero- and biotyping data, all V. cholerae strains tested had the major phenotypic and genotypic characteristics specific for the CL biotype. Antibiograms revealed the majority of the Bangladeshi strains to be resistant to trimethoprim-sulfamethoxazole, furazolidone, ampicillin, and gentamicin, while the Mexican strains were sensitive to all of these drugs, as well as to ciprofloxacin, erythromycin, and tetracycline. Pulsed-field gel electrophoresis (PFGE) of NotI-digested genomic DNA revealed characteristic banding patterns for all of the CL biotype strains although the Mexican strains differed from the Bangladeshi strains in 1 to 2 DNA bands. The difference was subtle but consistent, as confirmed by the subclustering patterns in the PFGE-based dendrogram, and can serve as a regional signature, suggesting the pre-1991 existence and evolution of the CL biotype strains in the Americas, independent from Asia.

Fig 1

Dendrogram showing genomic fingerprints of patterns of V. cholerae O1 CL biotype strains isolated in Mexico (1983 to 1997) and Bangladesh (1962 to 1989). The dendrogram was prepared by Dice similarity coefficient and UPGMA clustering methods by using PFGE images of NotI-digested genomic DNA; for this two PFGE gels were run under the same conditions. The scale bar at the top (left) indicates the correlation coefficient (%). The two major clusters separated the Mexican V. cholerae CL biotype strains (cluster B) from V. cholerae CL biotype strains isolated in Bangladesh (cluster A), suggesting a regional signature and independent evolution in the two geographically distinct ecosystems. INCL, Inaba classical; OGCL, Ogawa classical; Clin, clinical; Env. environmental.