Nalidixic acid screening test in detection of decreased fluoroquinolone susceptibility in Salmonella typhi isolated from blood.
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.


METHODS
Cross-sectional analytical study.


METHODS
Clinical Microbiology Laboratory of the Aga Khan Hospital, Karachi from January 2002 to December 2003.


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 mug nalidixic acid disc and nalidixic acid sensitive strains with mean zone of inhibition of 24.9 mm. All S. typhi isolates with ofloxacin MIC of capital ZHE, Cyrillic 0.125 microg/ml were found to be nalidixic acid resistant (MIC capital ZHE, Cyrillic32 microg/ml) whereas the isolates with ofloxacin MIC 0.06 microg/ml were nalidixic acid sensitive (MIC 8 microg/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 capital ZHE, Cyrillic 0.125 microg/ml).


CONCLUSIONS
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 25 mm around 5 microg ofloxacin disc is appropriate as a selection criterion to detect S. typhi isolates with reduced susceptibility to fluoroquinolones.
Zafar
Afia
A
Department of Pathology and Microbiology, The Aga Khan University Hospital, Karachi. afiazafar@aku.edu
Ibrahim
Nazish Gul
NG
Ahsan
Tanwir
T
Abbas
Zohair
Z
Zaidi
Anita
A
Hasan
Rumina
R
D000890:Anti-Infective Agents / Q000032:analysis / Q000494:pharmacology*; D003430:Cross-Sectional Studies; D024841:Fluoroquinolones / Q000494:pharmacology*; D006801:Humans; D008826:Microbial Sensitivity Tests; D009268:Nalidixic Acid / Q000032:analysis / Q000494:pharmacology*; D012485:Salmonella typhi / Q000187:drug effects*; D012680:Sensitivity and Specificity
D000890:Anti-Infective Agents; D024841:Fluoroquinolones; D009268:Nalidixic Acid