Molecular Characterization of Intermediate Susceptible Typhoidal Salmonella to Ciprofloxacin, and its Impact
Balaji Veeraraghavan1, Shalini Anandan2, Dhiviya Prabaa Muthuirulandi Sethuvel2
1Department of Clinical Microbiology, Christian Medical College, 8th Floor, Asha Building, Vellore, 632 004, Tamil Nadu, India. vbalaji@cmcvellore.ac.in.
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.
Insights
Ciprofloxacin resistance in Salmonella typhi is increasing, leading to treatment failures. Molecular analysis shows mutations in gyrA and parC genes correlate better with EUCAST resistance criteria, suggesting improved patient outcomes.
Area of Science:
- Microbiology
- Molecular Biology
- Infectious Diseases
Background:
- Extensive ciprofloxacin use has driven resistance in Salmonella typhi.
- Emergence of resistance leads to clinical treatment failures and delayed responses.
- Revised interpretative breakpoints by CLSI and EUCAST in 2012 necessitate further investigation.
Purpose of the Study:
- To molecularly characterize Salmonella typhi isolates with intermediate susceptibility to ciprofloxacin.
- To define the genetic basis of fluoroquinolone resistance in these isolates.
- To compare resistance mechanisms with CLSI and EUCAST criteria.
Main Methods:
- 33 typhoidal Salmonella isolates (27 S. typhi, 6 S. paratyphi A) were selected.
- Analysis included chromosomal genes (gyrA, gyrB, parC) and plasmid-mediated resistance genes (qnrA, qnrB, qnrS, aac(6')-lb-cr).
- Efflux pump activity for ciprofloxacin was assessed.
Main Results:
- A novel parC mutation (Glu84Gly) was identified in S. typhi in India.
- Only one isolate harbored the plasmid-mediated qnrB gene.
- 96% of isolates with intermediate minimum inhibitory concentration (CLSI) possessed gyrA and parC mutations.
- EUCAST criteria classified these isolates as resistant, unlike CLSI.
Conclusions:
- Molecular resistance mechanisms correlate better with EUCAST criteria than CLSI criteria.
- EUCAST criteria may lead to more appropriate therapy and reduced morbidity.
- Pefloxacin may serve as a surrogate marker for fluoroquinolone susceptibility testing.
