A Turbidimetric Study of the Responses of Selected Strains of Pseudomonas aeruginosa to Eight Antipseudomonal -Lactam Antibiotics

Abstract

Turbidimetric and morphologic responses to eight antipseudomonal β-lactam antibiotics were compared for selected strains of Pseudomonas aeruginosa with different susceptibilities to carbenicillin. In conventional minimal inhibitory concentration tests, all of the newer antibiotics appeared more active than carbenicillin, and apa1cillin and cefsulodin had the greatest overall activity. However, in turbidimetric tests the activity of apa1cillin and three other N-acyl penicillins (azlocillin, mezlocillin, and piperacillin) was inferior to that of carbenicillin and the other agents. The N-acyl penicillins were also all susceptible to intrinsic pseudomonal β-lactamase, so that dense bacterial populations inactivated these antibiotics in concentrations of >128 μg/ml during overnight incubation. Against carbenicillin-resistant strains, carbenicillin, ticarcillin, and sulbenicillin were the least active antibiotics, and cefsulodin had the best overall activity. Turbidimetric monitoring highlights the problems of interpreting the results of conventional minimal inhibitory concentration tests, particularly when large inocula are involved.