A Vibrio cholerae pathogenicity island associated with epidemic and pandemic strains

Abstract

The bacterial species Vibrio cholerae includes harmless aquatic strains as well as strains capable of causing epidemics and global pandemics of cholera. While investigating the relationship between pathogenic and nonpathogenic strains, we identified a chromosomal pathogenicity island (PAI) that is present in epidemic and pandemic strains but absent from nonpathogenic strains. Initially, two ToxRregulated genes (aldA and tagA) were studied and were found to be associated with epidemic and pandemic strains but absent in nontoxigenic strains. The region containing aldA and tagA comprises 13 kb of previously unidentified DNA and is part of a PAI that contains a regulator of virulence genes (ToxT) and a gene cluster encoding an essential colonization factor and the cholera toxin phage receptor (toxin-coregulated pilus; TCP). The PAI is 39.5 kb in size, has low %G؉C (35%), contains putative integrase and transposase genes, is f lanked by att sites, and inserts near a 10Sa RNA gene (ssrA), suggesting it may be of bacteriophage origin. We found this PAI in two clinical non-O1͞non-O139 cholera toxin-positive strains, suggesting that it can be transferred within V. cholerae. The sequence within this PAI includes an ORF with homology to a gene associated with the type IV pilus gene cluster of enteropathogenic Escherichia coli, a transposase from Vibrio anguillarum, and several ORFs with no known homology. As the PAI contains the CTX⌽ receptor, it may represent the initial genetic factor required for the emergence of epidemic and pandemic cholera. We propose to call this island VPI (V. cholerae pathogenicity island).In the last decade, the life-threatening diarrheal disease cholera has reached a wider distribution than at any other time in the 20th century. Cholera is caused by the bacterium Vibrio cholerae, which can be classified into over 140 serogroups (1). Prior to 1992, it was believed that only V. cholerae of the O1 serogroup were responsible for pandemic cholera and that strains of serogroups other than O1 were avirulent or caused only sporadic illness. However, in 1992 a O139 serogroup strain emerged and caused epidemic disease (2). The factors required for epidemic and pandemic ability are not fully understood, and there have been large outbreaks of cholera caused by toxigenic non-O1͞non-O139 strains that have not resulted in significant epidemic or pandemic disease (3, 4).Epidemic and pandemic strains of V. cholerae secrete cholera toxin (CT), the toxin responsible for the secretory diarrhea that is characteristic of the disease. CT is encoded by the ctxAB genes that are carried on a filamentous bacteriophage designated CTX⌽ (5). The bacterial receptor for this phage is the toxin-coregulated pilus (TCP), an essential colonization factor in human and animal models (6, 7). Expression of CT and TCP are coregulated by the ToxR regulatory system consisting of the proteins ToxR, ToxS, and ToxT (8, 9). Recently, it has been shown that the genes tcpP and tcpH within the TCP cluster also regulate virulence factors...

Characterization of pathogenicity island prophage in clinical and environmental strains of Vibrio cholerae

“…The implication from these results may be that: (i) some environmental strains could have acquired VPI phage; however, due to phages' size limitation they did not acquire the whole phage genomics; and (ii) the ability of V. cholerae to acquire, integrate and excise the VPI cluster is mediated by the VPI-encoded recombinase int and vpiT (Karaolis et al, 1998;Rajanna et al, 2003). Generation of amplicons with right and left junction primers suggests that VPIs have been integrated in the large chromosome of these isolates at sites identical to those of O1 strains (Fasano et al, 1991).…”

Comparison of antibiogram, virulence genes, ribotypes and DNA fingerprints of Vibrio cholerae of matching serogroups isolated from hospitalised diarrhoea cases and from the environment during 1997–1998 in Calcutta, India

“…Alternatively, even if the environment was the source, the clinical strains had apparently acquired extra elements in their genomes to differ signi®cantly in overall genotype from their environmental counterpart. According to Karaolis et al [36], in a species that has both pathogenic and nonpathogenic organisms, the pathogenic counterparts frequently possess one (or more) large genetic inserts known as`pathogenicity islands' which contain genes exclusively associated with the pathogenic phenotype.…”

ToxR interferes with CRP‐dependent transcriptional activation of ompT in Vibrio cholerae

“…In the other branch, independent of TcpP and ToxT, ToxR differentially regulates the expression of two outer membrane porins, OmpU and OmpT (Chakrabarti et al, 1996), resulting in almost exclusive expression of OmpU in a toxR + strain and exclusive expression of OmpT in a toxR -strain Crawford et al, 1998;Li et al, 2000). The genes encoding CT are part of the genome of a lysogenic phage (Waldor and Mekalanos, 1996), and the genes encoding TCP are part of the Vibrio pathogenicity island, which is also apparently of phage origin (Karaolis et al, 1998;. Therefore, it is generally believed that ToxR first evolved as the regulator of OmpU and OmpT expression in an ancestral V. cholerae strain, and subsequently gained the control of virulence genes when these genes were acquired through horizontal transfer.…”