Molecular Subtyping of Salmonella enterica Serovar Typhi by Pulsed-Field Gel Electrophoresis and Multiple-Locus Variable-Number Tandem-Repeat Analysis in India: Their Association with Antimicrobial Resistance Profiles

Molecular subtyping, and recently DNA sequencing based methods, which are commonly used for discriminating Salmonella enterica serovar Typhi (S. Typhi) isolates, leads to improved molecular epidemiological investigations for prevention and control of typhoid fever. We included S. Typhi blood isolates (n=66) from India during 2007-14 for molecular subtyping by pulsed-field gel electrophoresis (PFGE) and multiple-locus variable-number tandem-repeat (VNTR) analysis (MLVA) in association with antibiotic resistance profiles. Genotypic diversity was observed more in MLVA (Simpson’s index of diversity, D value 0.997) than PFGE (D value 0.864). Two prevalent pulsotypes containing nalidixic acid-resistant (NAL^R) and NAL^R-ciprofloxacin-resistant (CIP^R) S. Typhi isolates circulated in India. Multidrug-resistant (MDR), NAL^R-CIP^R and majority of NAL^R isolates were found to be clonal by PFGE. MLVA could differentiate the clonal isolates. Most of the MDR and NAL^R-CIP^R isolates showed variation in single or double VNTR loci, whereas NAL^R isolates varied in more than two loci reflecting higher genetic diversity among NAL^R isolates. Of the six VNTR loci, TR4699 (D value 0.838) and Sal02 (D value 0.890) loci played important roles as MLVA cluster supporting alleles. Rapid and high-level discriminatory power of MLVA may be useful for tracking and controlling the transmission of S. Typhi isolates during epidemiological investigations.

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