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

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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 β-lactamases, but all three compounds were more stable than were cefazolin, cephaloridine, and benzylpenicillin.

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