Susceptibility of Pseudomonas aeruginosa to Tobramycin or Gentamicin Alone and Combined with Carbenicillin

To explore more effective therapy for Pseudomonas aeruginosa, 264 recent clinical isolates were tested by agar dilution using gentamicin and tobramycin alone and combined with carbenicillin to seek synergistic effects. Synergism was defined as a fourfold or greater decrease in the minimal inhibitory concentration of each drug in a pair. At a concentration of 3.12 mug/ml, gentamicin inhibited 73% of the strains and tobramycin inhibited 98%. The gentamicin-carbenicillin combination was synergistically active against 57% of the strains, and tobramycin-carbenicillin was active against 46%. The effect did not correlate with either susceptibility or resistance to gentamicin or tobramycin alone. The data suggest that tobramycin or tobramycin plus carbenicillin may provide alternate therapy where susceptibility to gentamicin or synergism between gentamicin and carbenicillin cannot be demonstrated; however, the degree of susceptibility to either aminoglycoside antibiotic alone cannot be used to predict a synergistic effect.