Characterization of non-classical quinolone resistance in Salmonella enterica serovar Typhi: Report of a novel mutation in gyrB gene and diagnostic challenges

• Main Authors: Ruchi Gupta, Rajni Gaind, John Wain, Monorama Deb, Laishram Chandreshwor Singh, Seemi Farhat Basir
• Format: Article
• Language: English
• Published: Elsevier 2014-12-01
• Series: Biomolecular Detection and Quantification
• Subjects: Salmonella Typhi; Decreased ciprofloxacin susceptibility; DHPLC; gyrB mutation; VNTR
• Online Access: http://www.sciencedirect.com/science/article/pii/S2214753515000042

Objective: To establish the relative importance of Salmonella enterica serovar Typhi with non-classical quinolone resistance. Methods: Eight hundred and ninety-one isolates of S. Typhi, isolated between 2004 and 2011, were tested for antibiotic susceptibility determination using disc diffusion and E-test. The mechanisms of fluoroquinolone resistance were studied in a sub-set of the NALS (nalidixic acid susceptible) isolates by wave nucleic acid fragment analysis of PCR products from gyrA, gyrB, parC and parE and from the plasmid borne determinants: qnrA,B,S; aac(6′)-Ib-cr and qepA. To assess genetic relatedness multi-locus variable number tandem repeat analysis was carried out using five loci. Results: Eighty isolates with a nalidixic acid MIC of <32 mg/L (NALS) and a ciprofloxacin MIC of >0.064 mg/L CIPI (ciprofloxacin reduced susceptibility) were found. In 36 NALS CIPI isolates two distinct genotypes were identified when compared with 16 susceptible controls: Group B (n = 34), mutation in gyrB at codon 464, NAL MIC of 3–12 mg/L and CIP MIC of 0.064–0.5 mg/L.; and Group C, mutation in gyrA at codon 83 (n = 2) NAL MIC of 16 mg/L and CIP MIC of 0.25–0.38 mg/L. Group B isolates were found in different strain backgrounds as defined by MLVA. Conclusion: The use of nalidixic acid to screen for reduced susceptibility to fluoroquinolones in S. Typhi misses CIPI-NALS isolates, an established phenotype in India.