Evolutionary and functional analyses of variants of the toxin-coregulated pilus protein TcpA from toxigenic Vibrio cholerae non-O1/non-O139 serogroup isolates The GenBank accession numbers for the sequences reported in this paper are AY078355–AY078358.

Abstract

The toxin-coregulated pilus (TCP) is a critical determinant of the pathogenicity of Vibrio cholerae. This bundle-forming pilus is an essential intestinal colonization factor and also serves as a receptor for CTXφ, the filamentous phage that encodes cholera toxin (CT). TCP is a polymer of repeating subunits of the major pilin protein TcpA and tcpA is found within the Vibrio pathogenicity island (VPI). In this study genetic variation at the tcpA locus in toxigenic isolates of V. cholerae was investigated and three novel TcpA sequences from V. cholerae strains V46, V52 and V54, belonging to serogroups O141, O37 and O8, respectively, were identified. These novel tcpA alleles grouped into three distinct clonal lineages. The polymorphisms in TcpA were predominantly located in the carboxyl region of TcpA in surface-exposed regions of TCP fibres. Comparison of the genetic diversity among V. cholerae isolates at the tcpA locus with that of aldA, another locus within the VPI, and mdh, a chromosomal locus, revealed that tcpA sequences are far more diverse than these other loci. Most likely, this diversity is a reflection of diversifying selection in adaptation to the host immune response or to CTXφ susceptibility. An assessment of the functional properties of the variant tcpA sequences in the non-O1 V. cholerae strains was carried out by analysing whether these strains could be infected by CTXφ and colonize the suckling mouse. Similar to El Tor strains of V. cholerae O1, in vitro CTXφ infection of these strains required the exogenous expression of toxT, suggesting that in these strains ToxT regulates TCP expression and that these TcpA variants can serve as CTXφ receptors. All the V. cholerae non-O1 serogroup isolates tested were capable of colonizing the suckling mouse small intestine, suggesting that the different TcpA variants could function as colonization factors.

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

“…However, our results contradict the assumption that most cholera toxin-producing strains are also positive for TCP, since TCP is known to be the receptor for CTXφ infection of V. cholerae. In fact, our results appear to be consistent with data from recent studies that report the presence of the two virulence genes found in epidemic strains, ctxAB and tcpA, in environmental and clinical strain of serogroups other than O1 and O139 5,10,27,28,31,32 .…”

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

“…The findings reported here are supported by those of Karaolis et al (2001) who demonstrated that the central region of the VPI of tcpA contains most of the divergence between the VPIs of the 6th (classic) and the 7th (El Tor) pandemic strains. Previous studies by others have described that the genetic variants of the TCP island were carried by a number of environmental V. cholerae strains belonging to non-epidemic serogroups (Boyd & Waldor, 2002;Mukhopadhyay et al, 2001;Tay et al, 2008). Overall, the consensus all published reports have indicates mutational events in the tcp region.…”

Genetic diversity and virulence potential of environmental Vibrio cholerae population in a cholera-endemic area

“…Besides gene clusters associated with pandemic strains, recent studies have recognized the existence of different alleles of virulence genes in environmental V. cholerae strains, including different alleles of tcpA, tcpF, and toxT genes and different alleles of the CTX⌽ prophage repressor rstR in vibrios of various nonepidemic serogroups (9,10). It has been suggested that the different environmental alleles of virulence genes may have evolved in response to selective pressures that vary between the environment and the host (36). Because V. cholerae is a human pathogen whose natural habitat is the aquatic ecosystem, the existence of clinical and environmental alleles of different genes seems reasonable.…”