Background and objective: Extensive use of ciprofloxacin to treat Salmonella typhi infections has led to the emergence of resistance, resulting in clinical failure and delayed treatment response. Interpretative breakpoints for ciprofloxacin were revised by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) in 2012. Since the majority of S. typhi isolates fall under the category of 'intermediate susceptible' as per CLSI criteria, we undertook molecular characterization to better define the susceptibility of these isolates.
Methods: Of 113 typhoidal Salmonella isolates collected during 2014, 33 (27 S. typhi and 6 S. paratyphi A) were randomly selected to determine the presence of chromosomal (gyrA, gyrB and parC), plasmid (qnrA, qnrB, qnrS and aac(6')-lb-cr), and efflux-mediated fluoroquinolone resistance.
Results: To the best of our knowledge, the parC mutation Glu(84)-Gly was observed for the first time in S. typhi in India. Of 33 isolates, only one harbored the qnrB gene, which is responsible for plasmid-mediated resistance. No significant change in efflux pump activity was observed for ciprofloxacin, except one that showed a fivefold decrease. Ninety-six percent of isolates with intermediate minimum inhibitory concentration to ciprofloxacin (CLSI) had mutations in the gyrA and parC genes, which might translate to possible/probable clinical failure in patients if treated with ciprofloxacin. In contrast, the EUCAST criteria define these isolates as resistant and may result in appropriate therapy with reduced morbidity.
Conclusion: It was clear that the molecular mechanism of ciprofloxacin resistance correlates better with the EUCAST criteria than the CLSI criteria, which is also in agreement with the pefloxacin results, suggesting it as a surrogate marker for identifying fluoroquinolone susceptibility.