Characterization of nalidixic acid and ciprofloxacin resistance mechanisms in isolates from Bangladesh.The 38 isolates from Bangladesh were categorized into the following 5 ciprofloxacin susceptibility groups: a susceptible group, with MICs of ≤0.03 μg/ml; a group with reduced susceptibility, with MICs of 0.12 to 0.5 μg/ml; and resistant groups, with MICs of 4 μg/ml, 8 μg/ml, and 16 μg/ml (Table 3). The isolates with reduced susceptibility to ciprofloxacin were resistant to nalidixic acid (MICs of 128 to 256 μg/ml), and the trait of resistance was associated with a mutation in gyrase subunit A (S83F, S83Y, or D87N) and enhanced activity of the efflux pump for nalidixic acid. The isolates with a ciprofloxacin MIC of 4 μg/ml were highly resistant to nalidixic acid (MICs of ≥256 μg/ml), and the resistance was associated with a mutation (S83Y) in gyrase A, the presence of qnrS, and enhanced activity of the efflux pumps for ciprofloxacin and nalidixic acid. The isolates with ciprofloxacin MICs of 8 and 16 μg/ml also displayed high nalidixic acid resistance (MICs of >256 μg/ml), carried two mutations (S83F and D87G) in gyrase A and one mutation (E92K) in topoisomerase IV, and showed enhanced efflux pump activity for nalidixic acid. The isolates with a ciprofloxacin MIC of 16 μg/ml also showed enhanced efflux pump activity for ciprofloxacin.