[Comparative bactericidal activity of beta-lactam-aminoglycoside combinations against Pseudomonas aeruginosa]

The effects of combining each of 10 beta-lactamins (carbenicillin, ticarcillin, piperacillin, azlocillin, cefotaxime, moxalactam, ceftriaxone, cefoperazone, ceftazidime, cefsulodin) with 6 aminoglycosides were studied in vitro against two strains of Pseudomonas aeruginosa. The bactericidal activity was tested by two methods: solid medium technique of "cellophane transfer" and "checkerboard" method (broth-dilution micromethod partly automatic). No single antagonism was observed. Concerning new penicillins, the percentage of synergy is increasing from carbenicillin, to ticarcillin, to azlocillin and finally to piperacillin. For recent cephalosporins, the most synergistic combinations are obtained with ceftriaxone, cefoperazone and ceftazidime. If aminoglycosides associations are considered, there is a good activity whichever antibiotic chosen, with a higher level for streptomycin and lower one for tobramycin. Dibekacin combinations, also tested by the checkerboard method, are particularly synergistic with azlocillin and piperacillin for new penicillins, and equally well with ceftriaxone for recent cephalosporins.