Molecular epidemiology of Vibrio cholerae causing outbreaks & sporadic cholera in northern India

BACKGROUND & OBJECTIVES: Several outbreaks of cholera have been reported in Chandigarh region during a span of seven years from 2002-2008. The genetic characteristics of Vibrio cholerae isolates obtained during these outbreaks have not been adequately studied. The aim of this study was to do molecular typing of V. cholerae isolated from the sporadic and outbreak cases by pulsed-field gel electrophoresis (PFGE), Rep-PCR and ribotyping. METHODS: Fifty representative isolates of V. cholerae from outbreak as well as sporadic cases were subjected to molecular typing by PFGE, 173 isolates (163 clinical and 10 environmental) were typed by rep-PCR and ribotyping. Ribotyping was done by determination of rRNA restriction pattern of BglI restriction digestion and hybridization with 7.2 kb rRNA probe of pKK3535 plasmid using DIG DNA labelling and detection kit. Universal VC1 primer was used for rep-PCR. RESULTS: PFGE generated 15 pulsotypes, of which four matched the published pulsotypes and there were 11 new pulsotypes. PFGE was the most discriminatory method that could differentiate between isolates belonging to single ribotype. Pulsotype P1 corresponding to known pulsotype H1 was the major pulsotype till 2003. Pulsotype P3 corresponding to known pulsotype L emerged in 2004. The 2007 outbreaks in Punjab and Haryana were caused by P5 though P1 and P3 were isolated from the sporadic cases from the same region. The 2008 outbreak was caused by pulsotypes P6 and P7. Ribotype IV was the most predominant followed by RIII. This ribotype was not isolated after 2003 and ribotype IV became the most predominant 2004 onwards. Of the two unknown ribotypes (UNI and UN2), UNI was more common (27 isolates). Rep-PCR was the least discriminatory and divided all clinical isolates into four major profiles. The dendrogram analysis of PFGE revealed similarity of some clinical isolates with environmental isolates indicating the genetic relatedness. INTERPRETATION & CONCLUSION: Our findings showed that Rep-PCR was least discriminatory method. Ribotyping was a reliable and reproducible method. Ribotype IV was predominant ribotype followed by RIII. A total of 15 pulsotypes were generated and 11 of these were not reported earlier. Genetic relatedness was shown by clinical and environmental isolates which needs to be confirmed in future studies.