Abstract
Vibrio cholerae is ubiquitous in aquatic environments, with environmental toxigenic V. cholerae O1 strains serving as a source for recurrent cholera epidemics and pandemic disease. However, a number of questions remain about long-term survival and evolution of V. cholerae strains within these aquatic environmental reservoirs. Through monitoring of the Haitian aquatic environment following the 2010 cholera epidemic, we isolated two novel non-toxigenic (ctxA/B-negative) Vibrio cholerae O1. These two isolates underwent whole-genome sequencing and were investigated through comparative genomics and Bayesian coalescent analysis. These isolates cluster in the evolutionary tree with strains responsible for clinical cholera, possessing genomic components of 6th and 7th pandemic lineages, and diverge from “modern” cholera strains around 1548 C.E. [95% HPD: 1532–1555]. Vibrio Pathogenicity Island (VPI)-1 was present; however, SXT/R391-family ICE and VPI-2 were absent. Rugose phenotype conversion and vibriophage resistance evidenced adaption for persistence in aquatic environments. The identification of V. cholerae O1 strains in the Haitian environment, which predate the first reported cholera pandemic in 1817, broadens our understanding of the history of pandemics. It also raises the possibility that these and similar environmental strains could acquire virulence genes from the 2010 Haitian epidemic clone, including the cholera toxin producing CTXϕ.
Highlights
The exogenous introduction of toxigenic Vibrio cholerae O1 to Hispaniola putatively via United Nations peacekeepers in 2010 garnered global attention[1]
Despite ongoing aquatic environment monitoring for toxigenic V. cholerae by at least four groups[4,6,15,16], it is difficult to assess the true prevalence of this non-toxigenic O1 lineage in Haitian aquatic environment since the focus has largely been on identifying toxigenic strains
The current work identifies a V. cholerae O1 strain possessing Vibrio Pathogenicity Island (VPI)-1, a type 1 OS, and orphaned remnants of the CTX element. These strains lack the SXT/R391-family integrative conjugative element (ICE) found in 7th pandemic strains including the Haitian epidemic clone represented by 2010EL-1786
Summary
The exogenous introduction of toxigenic Vibrio cholerae O1 to Hispaniola putatively via United Nations peacekeepers in 2010 garnered global attention[1]. Non-toxigenic V. cholerae O1 strains have previously been reported in the Caribbean, South America, and Australia associated with clinical cases and the environment[12] These strains tend to phylogenetically cluster more closely with other O1 strains of the El Tor and classical lineage. Through ongoing monitoring of Haitian aquatic environments during the current epidemic, we identified two environmental non-toxigenic (ctxA/B negative) V. cholerae O1 strains possessing classical tcpA genes and genomically similar to the O395 classical strain[3]. To further investigate these relationships, the genomes were closed using PacBio long-read sequencing data and phylogenetically compared to representative clinical and environmental O1 and non-O1/O139 V. cholerae strains, including the recently published 2nd pandemic classical strain. Bayesian molecular clock analysis estimated the divergence date around 1548 C.E., consistent with the timeframe of the first European settlements in Hispaniola
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