Validity of nalidixic acid screening in fluoroquinolone-resistant typhoid salmonellae

To validate the screening of low-level fluoroquinolone resistance in typhoid salmonellae by using nalidixic acid (30 mg) disk providing an acceptable zone of inhibition.

Quasi-experimental study.

The Department of Microbiology, Armed Forces Institute of Pathology, Rawalpindi, Pakistan from July 2002 to June 2003.

Antimicrobial susceptibility of 225 clinical isolates of S. typhi (n=126) and S. paratyphi A (n=99) against nalidixic acid and ciprofloxacin was determined by the modified Kirby-Bauer disk diffusion and agar dilution techniques of NCCLS. The relationship between the zone sizes and the MICs of the two quinolones was plotted in the form of scattergrams and nalidixic acid MICs and zone of inhibition sizes were correlated with those of ciprofloxacin by regression analysis.

One hundred and ninety-five isolates were nalidixic acid-susceptible (MIC <16 microg/mL) and approximately 30 were nalidixic acid-resistant (MIC >32 microg/mL). All the nalidixic acid-susceptible isolates had ciprofloxacin MIC of <0.064 microg/mL. Among the nalidixic acid-resistant isolates approximately 20 had ciprofloxacin MIC > or =0.125 microg/mL and approximately 10 had ciprofloxacin MIC < or =0.03-0.064 microg/mL. The diameter of inhibition zone around a 30 mg nalidixic acid disk of nalidixic acid-resistant isolates was < or =13 mm (range 6-16 mm, mean 10.3 mm + SD 3.5 mm), while among nalidixic acid-susceptible isolates it ranged from 14 to 30 mm (mean 23.8 mm + SD 2.2 mm). The diameter of inhibition zone around a 5mg ciprofloxacin disk of nalidixic acid-resistant isolates ranged from 26 to 35 mm (mean 29.8 mm + SD 3.1 mm), while in nalidixic acid-susceptible isolates it ranged from 32 to 42 mm (mean 36.6 mm + SD 1.9 mm). With ciprofloxacin MIC > or =0.125 microg/mL taken as a breakpoint, a zone of

Screening for nalidixic acid resistance with a 30 microg nalidixic acid disk is a reliable and cost-effective method for detection of low-level fluoroquinolone resistance in typhoid salmonellae.