Quasi-experimental study.
PLACE AND DURATION OF STUDY: The Department of Microbiology, Armed Forces Institute of Pathology, Rawalpindi, Pakistan from July 2002 to June 2003.
MATERIALS AND 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%.
CONCLUSION: