Postantibiotic Effect of a Combination of Antimicrobial Agents on Pseudomonas aeruginosa

Treatment of severe Pseudomonas infections often calls for multidrug therapy. Combinations of aminoglycosides, beta-lactams and/or quinolones are generally administered, the effect of which may be additive, synergistic or indifferent. The present in vitro study was designed to find out the exact outcome of the postantibiotic effect (PAE). The minimal inhibitory concentrations and the PAE of three antibiotics (amikacin, ceftazidime and ciprofloxacin) were determined both singly and in combination. Ceftazidime alone exhibited a negative PAE but a synergistic effect was observed for the combination of ceftazidime with amikacin. The combination of ceftazidime with ciprofloxacin and ciprofloxacin with amikacin however gave an indifferent PAE. This synergistic PAE of amikacin with ceftazidime has a significant effect on designing optimal dosage regimens.