In Vitro Effects of Carbenicillin Combined with Gentamicin or Polymyxin B Against Pseudomonas aeruginosa

Disodium carbenicillin and gentamicin sulfate have both shown promise in the treatment of infections caused by Pseudomonas aeruginosa. This study was designed to explore possible synergistic relationships among the new as well as the established antimicrobial agents used to treat such infections. With an agar dilution technique, minimum inhibitory concentrations of 27 strains of P. aeruginosa were determined in two-dimensional tests. Graphs of equal biological activity (isobolograms) demonstrated moderate synergistic effects of the carbenicillin-gentamicin combination over therapeutically feasible concentration ranges. In contrast, the combination of carbenicillin and polymyxin B showed only additive or slightly antagonistic effects. Tests of bacterial killing confirmed the presence of carbenicillin-gentamicin synergy in 3 of 6 strains of P. aeruginosa, but did not show true antagonism between carbenicillin and polymyxin B. Clinical trials of both drug combinations are advisable to determine whether therapeutic results can be improved, and whether the dosages of gentamicin or polymyxin B can thereby be reduced to lessen their toxic hazards.