In vitro interactions of aminoglycosides with imipenem or ciprofloxacin against aminoglycoside resistant Acinetobacter baumannii.

The in vitro interactions between gentamicin, tobramycin, netilmicin and amikacin with imipenem and ciprofloxacin were evaluated by the killing curve technique against 20 clinical isolates of Acinetobacter baumannii highly resistant to aminoglycosides which were susceptible or moderately susceptible to imipenem and resistant or moderately susceptible to ciprofloxacin. Imipenem enhanced killing by gentamicin, tobramycin, netilmicin and amikacin in tests with 9, 12, 10 and 15 strains (45-75%) while ciprofloxacin with 3, 7, 5 and 6 strains (15-35%) respectively. Interaction results were influenced by the height of aminoglycoside minimum bactericidal concentrations (MBCs) but were independent of imipenem or ciprofloxacin MBCs and the presence of aminoglycoside modifying enzymes. It is concluded that enhanced killing after aminoglycoside interaction with imipenem or ciprofloxacin versus A. baumannii cannot be predicted but it should be carefully tested in vitro. The in vivo significance of the reported findings mandates clinical studies in humans.