[In vitro experiments on the working of combinations of gentamicin and beta-lactam antibiotics against Pseudonomas aeruginosa (author's transl)].

The M.B.C.'s of gentamicin and carbenicillin against Pseudonomas aeruginosa NCTC 10490 were measured under controlled conditions using a Biophotometer. The M.B.C. of gentamicin was 15 mug/ml but even in a concentration of 1,000 mug/ml carbenicillin was not bactericidal. In further experiments, subinhibitory concentrations of gentamicin (1 mug/ml) together with varying concentrations of carbenicillin were added to a log phase culture of the organism. Under these conditions the M.B.C. of carbenicillin was now 6 mug/ml. In tube dilution test the M.B.C. of carbenicillin alone was 15.6 mug/ml and for gentamicin 3.9 mug/ml. The M.B.C.'s of other beta-lactam antibiotics (ampicillin, penicillin G and cephalothin) were four to five times as great as for carbenicillin whereas that for ticarcillin was identical. Parallel to the "multiplication inhibition" test in the Biophotometer we investigated 51 strains of Pseudomonas aeruginosa freshly isolated from clinical material. Their M.B.C.'s were determined in tube dilution tests against doubling dilutions of beta-lactam antibiotics, with and without the addition of 1 mug/ml gentamicin. With this concentration of gentamicin, the M.B.C.'s of carbenicillin and ticarcillin were considerably lower than for these substances alone. In comparison to carbenicillin, ticarcillin was more effective against Pseudomonas aeruginosa. Our findings indicate that for Pseudomonas aeruginosa infections the combination of gentamicin with other beta-lactam antibiotics (ampicillin, penicillin G and cephalothin) is to be avoided. But the combination of gentamicin with either carbenicillin or ticarcillin appeared to be effective.