Characterising virulence in a nontoxigenic non-O1/non-O139 Vibrio cholerae isolate imported from Vietnam

<i>Vibrio cholerae</i> is a major human pathogen that can cause life-threatening acute diarrhea. <i>V. cholerae</i> are classified according to O-antigen polysaccharide outer membrane properties, where the serotypes O1 and O139 are strains that cause pandemics and epidemics while non-O1/non-O139 usually cause mild disease. The dynamic evolution of <i>V. cholerae</i> involves acquisition of new virulence factors through horizontal gene transfer and formerly nontoxigenic serogroups are increasingly being reported to cause severe forms of human disease. In this study we have serotyped one isolate (ST588-CPH) of imported <i>V. cholerae</i> from Vietnam to Denmark and performed whole genome sequencing to identify known virulence genes and furthermore studied the pattern of virulence in closely related pathogenic strains of <i>V. cholerae</i>. ST558-CPH was found to be a non-O1/non-O139 strain. Initial analysis from the whole genome sequencing gave a 96,6 % match to the O139-specific <i>wbfZ</i> gene, but in a second analysis with a higher identification threshold, the <i>wbfZ</i> gene was absent. We suggest a "de novo" display of a database misannotation, which explains the conflicting results. The MLST analysis revealed that the isolate belongs to the nontoxigenic non-O1/non-O139 sequence type ST558. ST558 has recently been reported as a sequence type forming a cluster of ST's that should be monitored, as it has shown to have virulence causing moderate to severe illness. Our analysis of virulence genes identified <i>MakA</i>, a recently discovered toxin, which seems to be generally present in both toxigenic and nontoxigenic strains.