ABSTRACT
BACKGROUND & OBJECTIVE: Production of extended spectrum beta-lactamases (ESBLs) and AmpC beta-lactamases are the most common mechanisms of antimicrobial resistance in Gram negative bacilli. A prospective study was undertaken to know the occurrence of ESBL and AmpC producing strains and their antibiotic susceptibilities to newer agents to guide empirical therapy for complicated urinary tract infections. METHODS: Over a period of five months (January to May 2003), organisms grown in pure culture and in significant numbers from urine sample were identified by standard biochemical tests and antibiotic susceptibility determined by disc diffusion method. Gram-negative bacilli that were resistant to third generation cephalosporins, ciprofloxacin and gentamicin/amikacin were defined as highly drug resistant uropathogens (HDRU). HDRU were further tested for ESBL and AmpC phenotypes. RESULTS: Uropathogens were isolated in significant numbers in 1979 (21.8%) of the total 9,072 samples, of which 438(22.1%) were HDRU. Two hundred and five consecutive HDRU isolates were tested for ESBL production and 36.5 per cent were found to be ESBL producers. The highest positivity was found to be in Klebsiella spp. (51.2%), followed by Escherichia coli (40.2%), Enterobacter aerogenes (33.4%) and Pseudomonas aeruginosa (27.9%). Both ESBL producers and non producers showed a high degree resistance to piperacillin (93.1 and 90.9%), amoxycillin-clavulanic acid (93.4 and 90.9%), aztreonam (79.4 and 78%), cefepime (76.7 and 78%), and ampicillin-sulbactam (76.7 and 70.4%). The most effective antibiotics for ESBL producers were imipenem (8.2% resistance), piperacillin-tazobactam (9.5%) and ceftazidime-clavulanic acid (23.2%). Among ESBL non-producers, piperacillin-tazobactam (31.06%), ceftazidime-clavulanic acid (49.2%) and imipenem (11%) were less effective when compared to ESBL producers. Fifty three piperacillin and piperacillin-tazobactam positive and 20 negative isolates were further tested for AmpC production and found that all 53 positive isolates were also positive by for AmpC beta-lactamase. INTERPRETATION & CONCLUSION: Overall, 22.1 per cent of our isolates were highly drug resistant, and ESBL producers could explain only 36.5 per cent of HDRU in our study. Therefore, we assume that AmpC beta-lactamases are more important in our setting. Based on our finding a test using discs containing piperacillin and piperacillin-tazobactam ( PtPc) disc at a distance of 20 mm would act as a useful screening procedure for AmpC production as AmpC beta-lactamase producers are more susceptible to tazobactam as compared to clavulanic acid.