Activity of gentamicin, tobramycin, polymyxin B, and colistimethate in mouse protection tests with Pseudomonas aeruginosa.

Mouse protection tests were carried out with four antibiotics and six strains of Pseudomonas aeruginosa. All strains were susceptible to all four antibiotics by an in vitro test. A heavier bacterial inoculum increased the mean effective dose of gentamicin and tobramycin, but not polymyxin B. Second and third doses of gentamicin in the mouse protection test made little change in the mean effective dose. In the mouse protection tests, tobramycin was the most active antibiotic if the results were analyzed in terms of the therapeutic index or ratio of toxicity to efficacy. Colistimethate was poorly inactive in vivo. Polymyxin B was most active on an absolute basis but also was the most toxic. One strain of Pseudomonas was classified as resistant to gentamicin in vivo although it was susceptible in vitro. Strains of Pseudomonas that were uniformly susceptible to antibiotics in vitro were not uniformly susceptible in the mouse protection test to low doses of antibiotic.