Synergy of azlocillin with aminoglycosides

The combination of azlocillin and the aminoglycosides amikacin, gentamicin, netilmicin, sisomicin, and tobramycin was synergistic. The FIC index was less than or equal to 0.5 for 28 to 42% of Pseudomonas aeruginosa isolates tested and with an additive FIC index greater than 0.5-1 for 53 to 63% of the isolates. Antagonism was not encountered. Isolates resistant to azlocillin (MIC greater than 100 mg/l) were more often synergistically inhibited by the combination of azlocillin and an aminoglycoside. The concentrations of antibiotics that synergistically inhibited Pseudomonas were those readily achieved in man. The combination of azlocillin and aminoglycosides prevented regrowth of isolates that grew after initial suppression by the antibiotics alone.