One hundred and thirteen clinical isolates of Pseudomonas aeruginosa from urinary tract infections were tested for their mechanisms of resistance to penicillins. Ninety-eight percent and 100.0% of the strains were resistant (MIC≧100 μg/ml) to ampicillin and penicillin G, respectively, while only 5.3% were resistant to piperacillin. Low permeability of the outer membrane, and penicillinase production were involved in their resistance mechanisms. Peptidoglycan synthesis in ether-treated cells of two representative strains was inhibited by ampicillin and piperacillin at the concentrations markedly lower than that for penicillin G
