Importance of methodology in determining bactericidal and bacteriostatic activities of azlocillin and ticarcillin against Pseudomonas aeruginosa.

The activities of azlocillin and ticarcillin against Pseudomonas aeruginosa were compared by estimating minimum inhibitory and bactericidal concentrations (MIC and MBC) in liquid and solid media, and by constructing killing curves from sequential viable counts. In MIC studies, azlocillin was about three times more active than ticarcillin in solid medium (agar dilution test) and in liquid media (tube and microdilution tests). When the MBC was measured, however, results varied according to the technique used. On agar and in microdilution tests, both azlocillin and ticarcillin were bactericidal, the MBC being 1.3-3 MIC. In the tube test, the MBC for ticarcillin was again about 3 MIC, but azlocillin appeared not to be bactericidal (MBC greater than 1 mg/ml). However, sequential viable counts of four clinical isolates showed that at 4 MIC both antibiotics reduced viable counts by a factor of 10(4) in 8 h. Our results stress the importance of methodology when assessing the antibacterial activity of an antibiotic.