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Research Paper |
Department of Microbiology, National University of Ireland, University College Cork, Cork, Ireland1
Howard Hughes Medical Institute and Division of Geographic Medicine and Infectious Diseases, Tufts-New England Medical Center and Tufts University School of Medicine, 750 Washington Street, Boston, MA 02111, USA2
Author for correspondence: E. Fidelma Boyd. Tel: +353 21 4903624. Fax: +353 21 4903624. e-mail: f.boyd{at}ucc.ie
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 CTX
, 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 CTX 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 CTX and colonize the suckling mouse. Similar to El Tor strains of V. cholerae O1, in vitro CTX 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 CTX 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.
Keywords: pathogenesis, intestinal colonization, CTXphi receptor
Abbreviations: CT, cholera toxin; TCP, toxin-coregulated pilus; VPI, Vibrio pathogenicity island
The GenBank accession numbers for the sequences reported in this paper are AY078355–AY078358.
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