Nalidixic acid susceptibility status of Salmonella enterica serovar Typhi isolates from Kolkata, India

Introduction and objectives: Typhoid is endemic in India; multidrug resistance of Salmonella enterica serovar Typhi has been reported from Kolkata, India, but nalidixic acid (Nx) resistance has not been documented in this part of the world. The aim of this study was to determine the Nx susceptibility for S. enterica serovar Typhi isolates (1991-2001) associated with enteric fever in Kolkata, India. Materials and methods: The clinical S. enterica serovar Typhi isolates (n=421) were subjected to Nx (the prototype quinolone, which is used for in vitro screening tests for fluoroquinolone resistance) susceptibility testing by disc diffusion and determination of minimum inhibitory concentration (MIC) values. The test results, from the two methods, were compared by scattergram analysis, and the sensitivity and specificity in determining Nx resistance by disc testing, with respect to MIC values, were calculated. Results: The S. enterica serovar Typhi isolates were categorized into resistant, intermediate and susceptible to Nx by disc testing and MIC. The isolates showed year-wise increment of Nx MICs (0.5-256g/ml) during 1991-2001. High sensitivity (100%) and specificity (92.96%) in determining Nx resistance by disc testing, compared to MIC values were obtained for the isolates. Conclusion: Increasing trend of Nx resistance, as determined by two in vitro methods, among S. enterica serovar Typhi isolates was noticed; such Nx resistance may help predict decreased susceptibility to ciprofloxacin (an anti-typhoid fluoroquinolone) among emerging S. enterica serovar Typhi in our part of the globe.