BETA-LACTAM RESISTANCE IN CLINICAL ISOLATES OF ESCHERICHIA-COLI CAUSED BY ELEVATED PRODUCTION OF THE AMPC-MEDIATED CHROMOSOMAL BETA-LACTAMASE

Among cephalothin-resistant isolates from patients with urinary tract infections, six E. coli strains were found to produce elevated amounts of a β-lactamase indistinguishable from that coded by the ampC gene of E. coli K-12. The resistance levels displayed by these isolates toward a number of β-lactams were, for five of them, considerably higher as compared with E. coli K-12 with the same amount of β-lactamase, implying the importance of intrinsic resistance in these isolates. Cefuroxime, and to a lesser extent cefamandole, were stable to hydrolysis by E. coli chromosomal β-lactamase but acted as inhibitors of the enzyme. Nevertheless, increased β-lactamase production mediated an increased resistance toward these drugs. No plasmids were found in the isolates, suggesting a chromosomal location for the respective ampC locus.