Amoxycillin/clavulanate resistance as related to β-lactamase content in Escherichia coli strains from community-acquired urinary tract infections in Greece

Objective: To determine the resistance rate to amoxycillin/clavulanate (AMC) in 100 Escherichia coli strains isolated from outpatients with urinary tract infection (UTI) in four Greek hospitals and assess the relationship between β-lactamase content and resistance to AMC.
Methods: Susceptibility to β-lactams was determined with the E-test. Sonic cell extracts were used as β-lactamase preparations. Conjugal transfer of resistance was performed in broth cultures. β-Lactamase quantities were evaluated by measuring nitrocefin hydrolysis. Isoelectric points (pls) of β-lactamases were determined by electrofocusing. The substrate specificity of the enzymes and the inhibitory activity of clavulanate were studied spectrophotometrically.
Results: Thirty-two isolates were resistant to ampicillin. Eight were resistant (MIC ≥ 32 mg/L) and 11 showed decreased susceptibility (MIC 4–16 mg/L) to AMC. The latter expressed at least four-fold higher amounts of TEM-1 β-lactamase compared with the TEM-1-producing AMC-susceptible isolates. Seven AMC-resistant isolates produced at least 16-fold higher amounts of TEM-1; in one isolate, resistance was attributed to an OXA-type β-lactamase. None of the AMC-resistant isolates was able to transfer resistance to AMC by conjugation. Clavulanate-resistant TEM variants were not detected.
Conclusions: Amoxycillin/clavulanate-resistant E. coli strains have become established in the Greek community. Resistance is mainly due to the production of large amounts of TEM-1 β-lactamase which is encoded from non-self-transmissible plasmids.