Susceptibility to penicillins and cephalosporins in beta-lactamase producing strains of E. coli and relative amount of beta-lactamase produced from these strains.

The genetic and physiological mechanisms for affecting the amount of chromosomally mediated beta-lactamase of Escherichia coli K 12 was reviewed. The presence of clinical E. coli isolates with elevated beta-lactamase production was assessed. Among cephalothin resistant isolates from patients with urinary tract infections, six E. coli strains were found to produce elevated amounts of a beta-lactamase indistinguishable from that coded by the ampC gene of E. coli K 12. The resistance levels displayed by these isolates towards a number of beta-lactams was very high as compared to E. coli strains being wild type for chromosomal beta-lactamase production. Cefuroxime and to lesser extent cefamandole were stable to hydrolysis by E. coli chromosomal beta-lactamase but acted as inhibitors of the enzyme. Nevertheless increased beta-lactamase production mediated an increased resistance towards these drugs.