Validity of nalidixic acid screening in fluoroquinolone-resistant typhoid salmonellae.
OBJECTIVE
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.


METHODS
Quasi-experimental study.


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


METHODS
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.


RESULTS
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 <or =33 mm around a 5 microg ciprofloxacin disk to detect low susceptibility strains had a sensitivity of 100% and a specificity of 82%. Screening for nalidixic acid resistance (inhibition zone diameter of < or =13 mm) in isolates with ciprofloxacin MIC 0.125 microg/mL using a 30 microg nalidixic acid disk yielded a sensitivity of 100% and a specificity of 95%.


CONCLUSIONS
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.
Butt
Tariq
T
Department of Pathology, Combined Military Hospital, Peshawar, Pakistan. tariqbut24@yahoo.com
Khan
Muhammad Yasin
MY
Ahmad
Rifat Nadeem
RN
Salman
Muhammad
M
Afzal
Raja Kamran
RK
D000890:Anti-Infective Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D024841:Fluoroquinolones; D006801:Humans; D008826:Microbial Sensitivity Tests; D009268:Nalidixic Acid / Q000494:pharmacology*; D015203:Reproducibility of Results; D012482:Salmonella paratyphi A / Q000187:drug effects*; D012485:Salmonella typhi / Q000187:drug effects*; D012680:Sensitivity and Specificity
D000890:Anti-Infective Agents; D024841:Fluoroquinolones; D009268:Nalidixic Acid