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Appl Environ Microbiol. 1969 June; 17(6): 783-786
Copyright © 1969 American Society for Microbiology. All Rights Reserved.

Effect of ß-Lactamase Location in Escherichia coli on Penicillin Synergy

¹ Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, New York 10032

ABSTRACT

Resistance to ampicillin in Escherichia coli is due generally to the presence of a ß-lactamase (penicillinase). Resistant strains have been found to fall into two groups: those with high-level resistance (1,000 µg/ml or greater) and those with low-level resistance (8 to 250 µg/ml). Most of the high-level resistant organisms posses ß-lactamases whose synthesis is episomally mediated. These strains release penicillinase from the cell when they are subjected to osmotic shock. Low-level resistant strains do not release the enzyme with osmotic shock. High-level resistant strains are not susceptible to the synergistic action of a penicillinase-resistant penicillin with ampicillin. Seventy eight per cent of low-level resistant strains are susceptible to the synergistic action of ampicillin and oxacillin. The two types of ß-lactamases are similar in regard to most properties; both enzymes are subject to competitive inhibition by penicillinase-resistant penicillins. The difference in location in the cell might explain why only some strains of E. coli are susceptible to the synergistic action of penicillin combinations.