Ofloxacin minimum inhibitory concentration versus disk diffusion zone diameter for Salmonella enterica serovar typhi isolates: Problems in the detection of ofloxacin resistance

Abstract

Using the guidelines of the National Committee for Clinical Laboratory Standards (NCCLS), a total of 421 blood culture isolates of Salmonella enterica serovar Typhi obtained during 1991 - 2001 were tested for susceptibility to ofloxacin (OFX) by the disc diffusion method, and for the determination of minimum inhibitory concentration (MIC) values of OFX by the agar dilution method. Among 421 isolates, 248 were fully OFX-sensitive showing MICs of 0.0125-0.075 μg/ml and inhibitory zone diameters of ≥24 mm. The remaining 173 isolates (MICs of 0.5-1.5 μg/ml) that were treated with OFX did not respond to the therapy. However, 169 (97.69%) of the 173 isolates were determined to be susceptible (zone diameter ≥16 mm) by the disc diffusion method, whereas only 3 were intermediately-susceptible (zone diameter 13-15 mm) and the final isolate showed OFX-resistance (zone diameter 12 mm). Thus, following the NCCLS guidelines, OFX-resistance in S. enterica serovar Typhi was not detected by the disc diffusion test. The present data suggest a revision of the NCCLS breakpoints in selecting OFX as the preferred treatment regimen for S. enterica serovar Typhi.