Conversion of a recA-Mediated Non-toxigenic Vibrio cholerae O1 Strain to a Toxigenic Strain Using Chitin-Induced Transformation

Abstract

Toxigenic Vibrio cholerae strains, including strains in serogroups O1 and O139 associated with the clinical disease cholera, are ubiquitous in aquatic reservoirs, including fresh, estuarine, and marine environments. Humans acquire cholera by consuming water and/or food contaminated with the microorganism. The genome of toxigenic V. cholerae harbors a cholera-toxin producing prophage (CT-prophage) encoding genes that promote expression of cholera toxin. The CT-prophage in V. cholerae is flanked by two satellite prophages, RS1 and TLC. Using cell surface appendages (TCP and/or MSHA pili), V. cholerae can sequentially acquire TLC, RS1, and CTX phages by transduction; the genome of each of these phages ultimately integrates into V. cholerae’s genome in a site-specific manner. Here, we showed that a non-toxigenic V. cholerae O1 biotype El Tor strain, lacking the entire RS1-CTX-TLC prophage complex (designated as RCT: R for RS1, C for CTX and T for TLC prophage, respectively), was able to acquire RCT from donor genomic DNA (gDNA) of a wild-type V. cholerae strain (E7946) via chitin-induced transformation. Moreover, we demonstrated that a chitin-induced transformant (designated as AAS111) harboring RCT was capable of producing cholera toxin. We also showed that recA, rather than xerC and xerD recombinases, promoted the acquisition of RCT from donor gDNA by the recipient non-toxigenic V. cholerae strain. Our data document the existence of an alternative pathway by which a non-toxigenic V. cholerae O1 strain can transform to a toxigenic strain by using chitin induction. As chitin is an abundant natural carbon source in aquatic reservoirs where V. cholerae is present, chitin-induced transformation may be an important driver in the emergence of new toxigenic V. cholerae strains.

Clinical and Environmental Vibrio cholerae Non-O1, Non-O139 Strains from Australia Have Similar Virulence and Antimicrobial Resistance Gene Profiles

“…Similar to our study, environmental NOVC strains SCE188, SCE200, SCE201, and SCE223 from India isolated in 1997/1998 lacked tcpA genes while ctxAB genes were present ( 38 ). In a previous in vitro study, the possibility of CTX gene transfer from a toxigenic O1 strain to an environmental NOVC strain was confirmed with uncertainty on whether the entire genome was transferred or whether the phage genome was integrated with the recipient chromosome upon transfer ( 39 ). According to our analysis, the entire CTX phage region is transferred from a toxigenic O1 El Tor strain from Australia to environmental NOVC strains.…”

Foodborne Pathogenic Vibrios: Antimicrobial Resistance

“…Recent studies showed that genetic basis of the emergence of multidrug resistant (MDR) and extensively drug resistant (ExDR) Vibrios and other enteric pathogenic bacteria is mainly due to horizontal gene transfer (HGT) through different types of highly dynamic mobile genetic elements, such as plasmids, integrating conjugative elements, superintegron, transposable elements, and insertion sequences ( Wozniak and Waldor, 2010 ; Kumar et al, 2017 ; Verma et al, 2019 ) that could potentially propagate from one bacteria to other closely or distantly related bacteria. Natural competency in Vibrios allows them to uptake and chromosomally integrate, exogenous DNA coming from multidrug resistant commensal gut bacteria and other sources which ultimately leads to the emergence of drug resistant Vibrios ( Meibom et al, 2005 ; Bag et al, 2019 ; Sinha-Ray et al, 2019 ). A novel “carry-back” mechanism for inter-phylum exchange of genes is also proposed where carrier DNA sequence may be transferred from Proteobacteria to Actinobacteria via conjugative plasmid or integron and then again taken up by Proteobacteria from Actinobacteria through transformation followed by genomic integration through homologous recombination.…”

Natural Transformation in a Classical-Biotype Vibrio cholerae Strain

“…1); the average length of DNA fragment taken up is 50 kbp (44). The conversion of V. cholerae O1 serogroup to O139 is suspected to have been caused by natural transformation, because it can be replicated in the laboratory by growth on chitin (45), and even toxigenic conversion may occur by chitin-induced transformation (46). As we have shown here, the superior ability of El Tor V. cholerae to undergo chitin-dependent natural transformation in comparison to classical V. cholerae (even with HapR restored) would be expected to provide an evolutionary advantage to this biotype.…”