Beta-lactamase patterns and beta-lactam/clavulanic acid resistance in Escherichia coli isolated from fecal samples from healthy volunteers

Fecal specimens from 50 healthy volunteers living in Izmir, Turkey, were ex amined for the presence of beta-lactamase producing Escherichia coli by sel ection on agar plates containing ampicillin (10 mg/L). Thirty-nine (78%) of the strains were ampicillin-resistant and ampicillin MIC50 values for thes e isolates were greater than or equal to 1024 mu g/ml (range 32- greater th an or equal to 1024 mu g/ml). Ampicillin MIC values remained above 64 mu g/ ml in 16 (41%) strains despite addition of clavulanic acid (2 mg/L). Beta-l actamase production of the clavulanate-resistant strains was further invest igated by analytical isoelectric focusing (pI). Enzymes with pIs of 5.4, 5. 6, 7.4, 7.6 and >8.5 were detected. Sixty-nine percent of the isolates prod uced a pI 5.4 enzyme that cofocused with TEM-1. Beta-lactamase assays revea led that hyperproduction of these enzymes was the predominant mechanism for clavulanate resistance. Twelve (75%) of the isolates were able to transfer their ampicillin resistance. The ampicillin and ampicillin plus clavulanic acid MIC values of all transconjugants were above 256 mu g/ml. Transferabl e ampicillin resistance was associated with resistance to other antibacteri als at the following frequencies: tetracycline 92%, trimethoprim 83%, strep tomycin 50%, gentamicin 25%, and chloramphenicol 8%, In conclusion, it has been suggested that commensal bacteria in normal popu lations make up the largest reservoir of antibiotic-resistant genes. Althou gh the exact molecular mechanisms could not be determined, the current stud y shows that the incidence of ampicillin and clavulanic acid resistance is also high in commensal fecal flora.