Characterization of a novel Vibrio pathogenicity island (VPI-2) encoding neuraminidase (nanH) among toxigenic Vibrio cholerae isolates.

Abstract

Acquisition of virulence genes encoded on mobile genetic elements has played an important role in the emergence of pathogenic isolates of Vibrio cholerae, the causative agent of the diarrhoeal disease cholera. The genes encoding cholera toxin (ctxAB), the main cause of profuse secretory diarrhoea in cholera, are encoded on a filamentous bacteriophage CTXphi. The toxin coregulated pilus (TCP), an essential intestinal colonization factor, was originally designated as part of a pathogenicity island named the Vibrio pathogenicity island (VPI), but this island has more recently been proposed to be the genome of a filamentous phage, VPIphi. In this study, it is shown that nanH, which encodes neuraminidase, maps within a novel pathogenicity island designated VPI-2. The 57.3 kb VPI-2 has all of the characteristic features of a pathogenicity island, including the presence of a bacteriophage-like integrase (int), insertion in a tRNA gene (serine) and the presence of direct repeats at the chromosomal integration sites. Additionally, the G+C content of VPI-2 (42 mol%) is considerably lower than that of the entire genome (47 mol%). VPI-2 encodes several gene clusters, such as a restriction modification system (hsdR and hsdM) and genes required for the utilization of amino sugars (nan-nag region) as well as neuraminidase. To determine the distribution of VPI-2 among V. cholerae, 78 natural isolates were examined using PCR and Southern hybridization analysis for the presence of this region. All toxigenic V. cholerae O1 serogroup isolates examined contained VPI-2, whereas non-toxigenic isolates lacked the island. Of 14 V. cholerae O139 serogroup isolates examined, only one strain, MO2, contained the entire 57.3 kb island, whereas 13 O139 isolates contained only a 20.0 kb region with most of the 5' region of VPI-2 which included nanH deleted in these strains.