Molecular Analyses of Vibrio cholerae O1 Clinical Strains, Including New Nontoxigenic Variants Isolated in Mexico during the Cholera Epidemic Years between 1991 and 2000

Abstract

We studied the evolution of Vibrio cholerae O1 during the 1991 to 2000 cholera epidemic in Mexico by biochemical, serological, and molecular characterization of strains collected during this period. Strains were divided into toxigenic and nontoxigenic groups according to the presence or absence of genes encoding cholera toxin. As previously reported, we characterized two populations among toxigenic strains, which were present from the first year of the epidemic. BglI rRNA analysis revealed that these strains had ribotype profiles, denoted M5 and M6 in our study, that were identical to those previously designated Koblavi B5 or Popovic 5 and Popovic 6a or Tamayo B21a, respectively. Ribotype M5 was isolated between 1991 and 1993. This ribotype had a low level of genetic variation as detected by pulsed-field gel electrophoresis (PFGE). Ribotype M6 persisted from 1991 to 2000. However, PFGE profiles suggested that two epidemiologically unrelated strains coexisted within this single ribotype from 1995 until the end of the epidemic. We identified three new BglI ribotypes, Mx1, Mx2, and Mx3, from nontoxigenic V. cholerae O1 strains isolated between 1998 and 2000; one of them grouped strains positive for the toxin-coregulated pilus island. They differed from nontoxigenic clones isolated in Latin America and on the U.S. Gulf Coast and are probably autochthonous Mexican V. cholerae O1 variants. Most of these new variants were isolated from states surrounding the Gulf of Mexico, where the highest incidence of cholera in the country was recorded. Thus, the Mexican Gulf Coast, like the U.S. Gulf Coast, may act as an environmental reservoir of V. cholerae O1.

Cholera between 1991 and 1997 in Mexico Was Associated with Infection by Classical, El Tor, and El Tor Variants of Vibrio cholerae

“…The association of V. cholerae with plankton in the coastal waters of Peru and Mexico has been well documented (7,8,18). Although V. cholerae strains isolated in Mexico after 1997 were not available for analysis in the present study, a recent report shows the involvement of serogroup O1 in endemic cholera in the Gulf of Mexico coast after 1997 (17). Although this publication does not include the full genetic characteristics of V. cholerae O1 strains isolated after 1997, it does indicate a change in the ribosomal pattern that separates strains into two different groups before and after 1997.…”

“…This is interesting, because the CL biotype caused the first six pandemics before being replaced by the ET biotype, and therefore, the CL biotype was considered to be extinct or to have at least disappeared as a cause of clinical disease. While the altered ET has already replaced the prototype 7th pandemic ET in Asia (11) and Africa (17,21), according to a recent study, the ET causing endemic cholera in Peru up until 2003 was of the 7th pandemic ET prototype (18). The present retrospective study of the phenotypic, molecular, and phylogenetic characterizations of V. cholerae O1 isolated in Mexico shows that the CL biotype was circulating in Mexico before and after 1991.…”

Vibrio cholerae Classical Biotype Strains Reveal Distinct Signatures in Mexico

“…The other strain, with an extra band of around 80 kb (designated pulsotype IIc), differed from all test strains including the CL reference strain, suggesting a unique molecular signature for this strain. The Mexican strain that matched in the PFGE pattern with the Bangladeshi strains cannot be clonal in the true sense because this strain had a distinctly different spatiotemporal origin and likely arose from genetically diverse V. cholerae populations involved with the 1991 epidemic and the subsequent endemic cholera in Mexico (2,19). Cluster analysis by dendrogram (UPGMA clustering method) of the gel images separated the V. cholerae CL biotype strains into two major clusters, A and B.…”

Pulsed-Field Gel Electrophoresis and PCR Characterization of Environmental Vibrio parahaemolyticus Strains of Different Origins

“…Persistence in the environment could also be a feature of toxigenic strains (i.e., those containing the trh gene), as cluster III connects trh-positive Ebro Delta strains isolated over the course of 4 years, which are characterized by little genetic diversity. Pulsotypes of trh-positive isolates were also found to be stable over time in Norway (8) and Italy (14), and this is also a common feature for other vibrios, such as V. cholerae (10).…”