Identification of β-Lactamase-Negative-Ampicillin Resistance Strains of Escherichia coli in 150 Isolates from Urinary Tract Infection and Fecal Flora in Kerman

β-lactamases are enzymes which inactivate the β-lactam antibacterial agents and are one of the major causes of resistance against these drugs. Recently there are reports on the isolation of bacteria which does not produce β-lactamase, but are resistant to penicillins. In the present study, β-laclamase production was determined using iodometric method on 150 ampicillin resistance Escherichia coli isolates from urinary tract infections (UTI) and fecal samples. These strains were resistance to both ampicillin and the combination of amoxicillin-clavulanic acid. They were identified as non-β-lactamase producers by iodometric method. Minimum inhibitory concentration for ampicillin in these strains were >512 µg/ml. This is the first report on the identification of ampicillin resistance β-lactamase negative E. coli in Iran.