Objective: To determine the validity of nalidixic acid screening test in the detection of high MICs of fluoroquinolone against Salmonella(S.) typhi isolated from blood and correlate zone diameters of ofloxacin with that of MIC value for nalidixic acid sensitive and resistant strains. Design: Cross-sectional analytical study. Place and duration of study: Clinical Microbiology Laboratory of the Aga Khan Hospital, Karachi from January 2002 to December 2003. Patients and methods: Two hundred S. typhi isolates from blood were included for nalidixic acid screening and ofloxacin susceptibility. Antibiotic susceptibilities for both the antibiotics were obtained by disc diffusion method whereas MICs were determined by standard agar dilution method as recommended by NCCLS guidelines. Sensitivity, specificity and correlation between both antimicrobial susceptibility methods were calculated and results expressed as scattergrams. Results: The results broadly classify S. typhi isolates into nalidixic acid resistant strains with no zone of inhibition around 30 μg nalidixic acid disc and nalidixic acid sensitive strains with mean zone of inhibition of 24.9mm. All S. typhi isolates with ofloxacin MIC of ≥0.125 μg/ml were found to be nalidixic acid resistant (MIC ≥82 μg/ml) whereas the isolates with ofloxacin MIC ≤0.06 μg/ml were nalidixic acid sensitive (MIC ≤8 μg/ml). Screening for nalidixic acid resistance was found to be 100% sensitive and 97% specific in identifying S. typhi strains with reduced susceptibility to fluoroquinolone (MIC ≥0.125 μg/ml). Conclusion: Nalidixic acid resistance as a screening method is proved to be significant in identifying S. typhi isolates with reduced susceptibility to fluoroquinolones. It is also suggested that inhibition zone of ≤25mm around 5μg ofloxacin disc is appropriate as a selection criterion to detect S. typhi isolates with reduced susceptibility to fluoroquinolones.
|Number of pages||5|
|Journal||Journal of the College of Physicians and Surgeons--Pakistan : JCPSP|
|Publication status||Published - Jul 2005|
- Fluoroquinolone resistance
- Nalidixic acid screening blood isolates
- Salmonella typhi