Abstract
Vibrio cholerae is commonly found in estuarine water systems. Toxigenic O1 and O139 V. cholerae strains have caused cholera epidemics and pandemics, whereas the nontoxigenic strains within these serogroups only occasionally lead to disease. To understand the differences in the genome and clonality between the toxigenic and nontoxigenic strains of V. cholerae serogroups O1 and O139, we employed a whole genome PCR scanning (WGPScanning) method, an rrn operon-mediated fragment rearrangement analysis and comparative genomic hybridization (CGH) to analyze the genome structure of different strains. WGPScanning in conjunction with CGH revealed that the genomic contents of the toxigenic strains were conservative, except for a few indels located mainly in mobile elements. Minor nucleotide variation in orthologous genes appeared to be the major difference between the toxigenic strains. rrn operon-mediated rearrangements were infrequent in El Tor toxigenic strains tested using I-CeuI digested pulsed-field gel electrophoresis (PFGE) analysis and PCR analysis based on flanking sequence of rrn operons. Using these methods, we found that the genomic structures of toxigenic El Tor and O139 strains were syntenic. The nontoxigenic strains exhibited more extensive sequence variations, but toxin coregulated pilus positive (TCP+) strains had a similar structure. TCP+ nontoxigenic strains could be subdivided into multiple lineages according to the TCP type, suggesting the existence of complex intermediates in the evolution of toxigenic strains. The data indicate that toxigenic O1 El Tor and O139 strains were derived from a single lineage of intermediates from complex clones in the environment. The nontoxigenic strains with non-El Tor type TCP may yet evolve into new epidemic clones after attaining toxigenic attributes.
Highlights
Vibrio cholerae is autochthonous to warm aquatic ecosystems [1]
The cholera toxin gene ctxAB is located in the lysogenic bacteriophage, CTXW, on the Vibrio genome [2]. ctxAB can be transferred between toxigenic strains and nontoxigenic strains by lysogenic infection with CTXW, which is facilitated by the receptor, toxin coregulated pilus (TCP) [5]
We analyzed the genomic structures of the V. cholerae serogroups O1 and O139 using both WGPScanning and comparative genomic hybridization (CGH) approaches
Summary
More than 200 O-antigen serogroups of V. cholerae have been identified; only serogroups O1 and O139 strains have been associated with cholera epidemics and pandemics [2]. Epidemics of the O139 cholera emerged in 1992 in Bangladesh and India [3,4]. This group currently remains confined to Asia. The cholera toxin gene ctxAB is located in the lysogenic bacteriophage, CTXW, on the Vibrio genome [2]. CtxAB can be transferred between toxigenic strains and nontoxigenic strains by lysogenic infection with CTXW, which is facilitated by the receptor, toxin coregulated pilus (TCP) [5]. Each variant of TCP can mediate infection of certain nontoxigenic strains by different types of CTXW at varying efficiency [6]
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