[Combined in vitro effect of new betalactam antibiotics and aminoglycosides against Pseudomonas aeruginosa ].

The association of 11 betalactam antibiotics (carbenicillin, ticarcillin, azlocillin, piperacillin, mezlocillin, cefotaxim, cefoperazone, ceftriaxon, moxalactam, cefsulodin, ceftazidim) with each of the 3 aminoglycosides (gentamicin, tobramycin, amikacin) were studied by the broth dilution method ("checkboard" technic) against 6 strains of Pseudomonas aeruginosa chosen as a function of their "phenotype". The coefficient of synergy (FIC and FBC) were calculated for the 198 combinations (33 combinations per strain). Bacteriostatic synergy was observed in 18 per cent of the cases and bactericidal synergy in 48 per cent. Synergic association were found to be a function of phenotype. Synergy was rarely observed against bacterial strains resistant to aminoglycosides. Synergy was most frequently observed with two antibiotics in intermediate potency, or an active aminoglycosides combined with an active betalactam antibiotic. The combination containing amikacin produced the most synergy.