-Lactamase and the Resistance of Pseudomonas aeruginosa to Various Penicillins and Cephalosporins

Abstract

SUMMARY: Eight strains of Pseudomonas aeruginosa, induced by penicillin G, produced different amounts of β-lactamase. Significant enzyme activity appeared within 5 min. and its highest value was obtained 3 h. after the addition of penicillin G. The production of the enzyme was arrested shortly after removal of the inducer. High concentrations of penicillin G, other penicillins or cephalosporins were necessary for optimal induction. 6-Amino-penicillanic acid (6-APA) was the best inducer tested, while cephalothin was the worst. Most of the enzyme was intracellular after conversion of the cells to spheroplasts in the presence of carbenicillin. The structure of both the nucleus and side chain of the penicillins and cephalosporins determined the rate of their hydrolysis by the β-lactamase. Generally, the enzyme was more active against 7-aminocephalosporanic acid (7-ACA) derivatives than against 6-APA derivatives, except cephalexin and cephaloglycin, which were relatively resistant to hydrolysis. No relation was found between the sensitivity of the pseudomonads to penicillin G and the amount of the enzyme in their cell-free preparations. The sensitivity of the different antibiotics to hydrolysis by the enzyme was not a major determinant in the resistance of Pseudomonas aeruginosa 1978 to them. These results indicate that the resistance of the bacteria to the β-lactamase-sensitive penicillins and to cephalosporins is dependent on a combined effect of β-lactamase and on an intrinsic resistance, while the resistance of the bacteria to the β-lactamase-resistant penicillins depends on the intrinsic resistance alone.