Studies on the susceptibility of 150 consecutive clinical isolates of Pseudomonas aeruginosa to tobramycin, gentamicin, colistin, carbenicillin, and five other antimicrobials.

The in vitro susceptibilities of 150 clinical isolates of Pseudomonas aeruginosa to the aminoglycoside tobramycin and eight other antimicrobials were evaluated. The Food and Drug Administration standardized disk diffusion method showed that tobramycin inhibited 100% of the 150 isolates with gentamicin inhibiting only 90.7%. The difference between colistin (97.3%) and tobramycin was less marked. Carbenicillin (87.3%) was found to be slightly less active than gentamicin. Only a small percentage of the isolates were inhibited by the other drugs used. By using a minimal inhibitory concentration of 4 mug/ml in brain heart infusion broth as the level of susceptibility, it was found that only three of the isolates showed resistance to tobramycin, whereas 20 of the isolates were resistant to gentamicin. Approximately half of the isolates were inhibited by 0.5 mug/ml or less of tobramycin, whereas 1.5 mug of gentamicin per ml was required to inhibit 50% of susceptible strains.