Studies of in vitro synergy between several beta-lactam and aminoglycoside antibiotics against endocarditis strains of Pseudomonas aeruginosa.

Ten strains of Pseudomonas aeruginosa isolated from patients with endocarditis (1969-1975) and eight similar strains (1980) were assayed for minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) to several aminoglycosides (gentamicin, tobramycin, amikacin) and beta-lactam antibiotics (ticarcillin, piperacillin, azlocillin, moxalactam and MKO 787). In vitro synergy (1969-1975 series) between beta-lactam and aminoglycoside antibiotics was shown uniformly with azlocillin (100 per cent) followed by moxalactam (80 per cent), piperacillin and ticarcillin (66 per cent) and MKO 787 (13.3 per cent). Results were similar in 1980. Synergy of azlocillin was demonstrated with five strains previously not showing synergy between carbenicillin and an aminoglycoside. In 1980 four of eight patients infected with pseudomonads that were not synergistically affected in vitro were refractory to treatment with the piperacillin-aminoglycoside combination. In vitro synergy of the infecting strain is necessary for successful medical treatment of patients with P. aeruginosa infective endocarditis.