Antibiotic susceptibility pattern of Salmonella enterica serovar typhi and Salmonella enterica serovar paratyphi A with special reference to quinolone resistance

Background and Objectives: Typhoid fever is endemic in India. Extensive use of first-line antibiotics has led to the emergence of multi-drug resistant (MDR) Salmonella typhi. Ciprofloxacin has become empirical therapy of choice against MDR salmonellae. Recent year′s emergence of low-level ciprofloxacin resistance in salmonellae resulted in delayed response and serious complications. Nalidixic acid (NA) screen test is used as surrogate marker for detection low-level ciprofloxacin resistance. In this study, we evaluated prevalence of MDR and low-level ciprofloxacin resistant S. typhi and Salmonella paratyphi A. Materials and Methods: A total of 50 blood culture isolates of S. typhi and S. paratyphi A were tested for antibiotic susceptibility according to Clinical Laboratory Standards Institute (CLSI) method. Minimal inhibitory concentration (MIC) to ciprofloxacin was carried out by E-test and agar dilution method. Results: Among the 50 salmonella isolates, 80% were S. typhi and 20% were S. paratyphi A. MDR was found in 2% S. typhi. NA resistant salmonellae showed ciprofloxacin MIC ranging from 0.25 to 0.75 μg/ml. One isolate of S. typhi showed ciprofloxacin MIC of 32 μg/ml and was also resistant to ceftriaxone. NA screen test for low-level ciprofloxacin resistance was 100% sensitive and 97.9% specific. Interpretation and Conclusion: NA resistant isolates should be tested for ciprofloxacin MIC to decide therapeutic options. The current CLSI breakpoints may have to be re-evaluated for salmonellae.