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

ABSTRACT

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. Cross-sectional analytical study. Clinical Microbiology Laboratory of the Aga Khan Hospital, Karachi from January 2002 to December 2003. 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. The results broadly classify S. typhi isolates into nalidixic acid resistant strains with no zone of inhibition around 30 micro g 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 micro g/ml were found to be nalidixic acid resistant [MIC >/= 32 micro g/ml] whereas the isolates with ofloxacin MIC nalidixic acid sensitive [MIC 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 micro g/ml]. 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 ofloxacin disc is appropriate as a selection criterion to detect S. typhi isolates with reduced susceptibility to fluoroquinolones