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Antimicrob Agents Chemother. 1977 January; 11(1): 1-6
Copyright © 1977 American Society for Microbiology. All Rights Reserved.

Comparative In Vitro Activity of Pirbenicillin, Ticarcillin, and Carbenicillin Against Pseudomonas aeruginosa

Section on Infectious Diseases, Department of Medicine, University of Minnesota Medical School, Minneapolis, Minnesota 55455

ABSTRACT

Pirbenicillin, a semisynthetic penicillin, showed greater in vitro activity against 68 recent isolates of Pseudomonas aeruginosa than did ticarcillin or carbenicillin. The median minimum inhibitory concentration of each of these three compounds, respectively, was 3.1, 12.5, and 25 µg/ml when a 10^–4 dilution of an overnight culture (about 10⁵ colony-forming units [CFU]/ml) was used as inoculum, but these differences were less striking when larger inocula were used: at 10⁷ CFU/ml these values were 6.25, 12.5, and 50 µg/ml, and at 10⁸ to 10⁹ CFU/ml these values were 50, 50, and 100 µg/ml. All three compounds showed greater inhibitory activity at pH 6 than at pH 8. This pH effect was greatest with pirbenicillin, for 6.25 µg of pirbenicillin per ml inhibited 7, 11, and 57% of the strains at pH 6, 7, and 8, respectively; these values were 4, 4, and 11% with ticarcillin and 0, 0, and 7% with carbenicillin. At sufficient inhibitory concentrations, the rates of bacterial killing of the three compounds were similar. The observed differences in anti-pseudomonad activity were not due to differences in stability to pseudomonad ß-lactamases, but all three compounds were more stable than were cefazolin, cephaloridine, and benzylpenicillin.