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

Abstract

The DNA sequences of the asd genes from 45 isolates of Vibrio cholerae (19 clinical O1 isolates, 2 environmental nontoxigenic O1 isolates, and 24 isolates with different non-O1 antigens) were determined. No differences 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 outgroup suggest that the sixth-pandemic, seventhpandemic, and U.S. Gulf isolates are three clones that have evolved independently from different lineages of environmental, nontoxigenic, non-O1 V. cholerae isolates. There is evidence for horizontal transfer of O antigen, since isolates with nearly identical asd sequences had different O antigens, and isolates with the O1 antigen did not cluster together but were found in different lineages. We also found evidence for recombination events within the asd gene of V. cholerae. V. cholerae may have a higher level of genetic exchange and a lower level of clonality than species such as Salmonella enterica and Escherichia coli.Vibrio cholerae is a normal inhabitant of aquatic environments (46). However, contaminated water supplies in some areas of the world have enabled some clones of the species to become pathogenic for humans. Seven pandemics of cholera have been recorded previously (12,36). The seventh pandemic, beginning in 1961, is clonal (20,22,24,37), is well defined, and has a known starting date, and previous studies (7,22,24,37,53) have shown it to have accumulated variation, with both random genetic drift and recombination possibly being involved. There are 140 known serotypes of V. cholerae (50). Until recently, all recorded pandemic and epidemic cases of cholera have been associated with the type O1 antigen; however, an epidemic which began in India late in 1992 and spread to several neighboring countries is caused by an O139 strain (1,8,33,39). Studies have shown that the O139 strain is genetically very similar to strains of the seventh pandemic (13,22,37,54). Karaolis et al. (22) have further shown that O139 isolates appear to have evolved from early seventh-pandemic isolates. Biochemically, it is nearly impossible to distinguish O1 and non-O1 V. cholerae (10,38). V. cholerae non-O1 strains are far more frequently isolated from environmental sources than O1 strains and appear to constitute part of the normal microflora of prawns (31) and oysters (2,4,28,47). Most environmental isolates do not possess cholera toxin genes, a heat-stable enterotoxin (31, 32), or toxin coregulated pilus (49, 54).The population structure and molecular evolution of V. cholerae as a species have not been well characterized, and the relationship of the sixth-and seventh-pandemic isolates is not clear. We have investigated sequence variation within V. cholerae by examining variation in the asd locus, one of the few V. cholerae chromosomal housekeeping genes sequenced to date. Housekeeping genes are conce...

Genomic characterization of non-O1, non-O139 Vibrio cholerae reveals genes for a type III secretion system

“…These studies revealed a surprisingly high degree of genomic conservation among TCP-positive O1 classical, O1 El Tor, and O139 strains that were previously thought to be quite diverse based on other methodologies (6,17,26). The microarrays also identified two chromosomal islands that are uniquely present in O139 and El Tor O1 seventh pandemic strains, suggesting that these regions may encode factors involved in the endemic and pandemic potential of these clones (25).…”

“…In contrast, environmental non-O1, non-O139 strains have typically been reported to be quite diverse. These conclusions were reached by using a variety of techniques, including ribotyping (22), amplified fragment length polymorphism (3), pulsed-field gel electrophoresis (3), and comparative nucleotide sequence analysis (6,52). These analytical methods are useful in assessing relatedness of strains but are limited in their ability to define gene content differences that might have biological and phylogenetic significance.…”

“…Gray arrows indicate genes encoding hypothetical proteins. closely related and constitute a globally distributed clone (6,27,51). A few pathogenic strains of non-O1, non-O139 serogroups such as O141, O10, and O12 have been isolated in outbreaks of gastroenteritis, and similarly show a clonal structure (9-11).…”

Distribution of virulence markers in clinical and environmental Vibrio cholerae non-O1/non-O139 strains isolated in Brazil from 1991 to 2000

“…This finding is based first on the emergence of serogroup O139 (Bengal) in Bangladesh (CT positive) and Argentina (CT negative), each of which clearly evolved independently 33 . Second, the isolates from the sixth and seventh pandemic, and U.S. Gulf Coast belong to three different clones, each independently, evolved from environmental non-O1 V. cholerae isolates 16 . Third, the high relatedness of different serogroups of V. cholerae strains isolated in Brazil 34 , and other epidemic serogroups have emerged, including V. cholerae O10 13 Cento e setenta e nove amostras de V. cholerae não O1/não O139, isoladas de casos clínicos (139) e de meio ambiente (40), no período de 1991 a 2000 no Brasil, foram caracterizadas antigenicamente pelo National Institute of Health (Japão) e investigadas quanto ao seu potencial genético de virulência, representado pelos genes ctxA, zot, ace e tcpA.…”

“…The specie has been divided into serogroups O1 and non-O1 which differ each other for their ability to cause cholera epidemics 7 . Prior to 1992, it was believed that only V. cholerae O1 strains were responsible for pandemics of cholera and all major epidemics 16 , and the non-O1 strains were non virulent responsible for sporadic cases of gastroenteritis 26 and extra-intestinal infections 24 . In 1992, an outbreak of cholera in India and Bangladesh, which subsequently spread into several parts of the subcontinent, was caused by a novel non-O1 strain: V. cholerae O139 Bengal 1 .…”