ABSTRACT
The bacteriostatic activity of ampicillin-amikacin and ampicillin-gentamicin combinations was shown to be synergistic against the tested strain of Pseudomonas aeruginosa, fractional inhibitory concentration [FIC] index was 0.25. Pre-incubation of all tested aminoglycosides [gentamicin, tobramycin, kanamycin, amikacin and streptomycin] with the crude pseudomonal beta-lactamase, revealed neither inhibition nor stimulation of such enzyme. Beta-lactam antibiotics, especially ampicillin and benzyl penicillin, inhibited the activity of aminoglycoside-inactivating enzymes produced by Pseudomonas aeruginosa. The tested aminoglycosides and beta-lactams at subinhibitory concentrations were noninducible to beta-lactamase and aminoglycoside inactivating enzymes, respectively. Aminoglycoside-beta-lactam synergism against Pseudomonas aeruginosa was suggested to be in part due to aminoglycoside-inactivating enzyme inhibition by beta-lactams