Salmonella enterica serovar typhi: molecular analysis of strains with decreased susceptibility and resistant to ciprofloxacin in India from 2001-2003.

Chromosomally-mediated reduced susceptibility to ciprofloxacin narrows the therapeutic options in enteric fever. We made a molecular comparison of clinical isolates of fluoroquinolone-resistant strains of Salmonella enterica serotype Typhi from January 2001 to May 2003; 178 isolates were subjected to antimicrobial susceptibility testing by the Kirby-Bauer method of disk diffusion, and agar dilution was used to determine the minimum inhibitory concentration (MIC) to ciprofloxacin. Nalidixic-acid resistant strains (NARST) were observed in 51% of the isolates, of which 98.9% had decreased susceptibility (MIC> or =0.125-1 microg/mL) to ciprofloxacin. A single strain (4 microg/mL) was resistant to ciprofloxacin and double mutations were found in the gyrA gene (76 Asp->Asn, 44 leu->Ileu). Among seven NARST strains with reduced susceptibility, a single mutation was found in five strains, one of which had 76 Asp->Asn and two each had mutations at 87 Asp->Asn and 72 Phe->Tyr, respectively); no mutations could be detected in two isolates. Routine antimicrobial surveillance, coupled with molecular analysis of fluoroquinolone resistance, is crucial for revision of enteric fever therapeutics.