Knowledge of antimicrobial resistance patterns in E. coli, the predominant pathogen associated with urinary tract infections (UTI) is important as a guide in selecting empirical antimicrobial therapy. Methods: To describe the antimicrobial susceptibility of E. coli associated with UTI in a major university hospital in Tehran (Iran), seventy-six clinical isolates of E. coli were studied for susceptibility to b-lactam antibiotics by the disc diffusion method and Minimal Inhibitory Concentrations determination. Results: All isolates were resistant to ampicillin, amoxicillin and oxacillin. Resistance to the other tested antibiotics was shown to be 93.4% to cefradine, 76.3% to carbenicillin, 47.3% to cefazoline, 50% to cefalexin and 32.8% to cephalothin while 1.3% expressed resistance to cefoxitime, and 2.6% were resistant to ceftizoxime and ceftriaxone. Two isolates (2.4%) harbored extended spectrum b-lactamases (ESBL) shown by the double disc diffusion method. Substrate hydrolysis by ultra violet spectroscopy showed that 87.4% harbored penicillinases, 9% produced cephlosporinases and 3.6% degraded both substrates. Clavulanic acid inhibited enzyme activity in 82.9%, of which 78.95% was penicillinases (group IIa) and 3.95% was cephalosporinases (group IIb) of the Bush classification system. The rest of the isolates (6.58 %) were placed in group IV b-lactamases. No group III b-lactamase was found, as EDTA inhibited none of the enzymes. DNA amplification by polymerase chain reaction using specific primers for ampC, TEM and SHV type b-lactamases for all of the isolates showed that 47 organisms (60%) carried the TEM gene and 18 isolates (24%) harbored blaTEM and ampC genes. About 26% of the organisms harbored SHV type enzymes. Conclusion: These results indicate that E. coli can posses a variety of b-lactamases that are responsible for β-lactam resistance.

Keywords: Urinary, β-lactamase, TEM, SHV, Amp C, E. coli

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