The Carbenicillin-Gentamicin Combination AgainstPseudomonas aeruginosa

The inhibitory activity of carbenicillin and gentamicin alone and in combination was evaluated against 130 clinical isolates of Pseudomonas aeruginosa, in concentrations that can be reliably produced in the serum of patients. The antibiotic combination had an increased inhibitory effect against 114 isolates, when compared with the most effective drug alone. Minimum inhibitory concentrations of gentamicin were less than 40 µg/ml, and a zone of inhibition was produced around the 10-µg gentamicin sensitivity disc for 112 of these 114 isolates. The remaining 16 isolates were not inhibited any better by the combination than by the most effective antibiotic alone. The inhibitory concentrations of carbenicillin for these 16 isolates varied, but each was highly resistant to gentamicin (≥80µg/ml). Fifteen of these 16 isolates produced no zone of inhibition around the 10-µg gentamicin disc. The absence of this zone of inhibition identifies most highly resistant isolates (against which the combination has no increased effect in vitro) and can suggest a need for a change in therapy.