Synergy of imipenem--a novel carbapenem, and rifampin and ciprofloxacin against Pseudomonas aeruginosa, Serratia marcescens and Enterobacter species.

Although imipenem inhibits most bacteria at very low concentrations, some Pseudomonas aeruginosa, Serratia marcescens and Enterobacter species are resistant or become resistant after exposure. At concentrations of rifampin equivalent to those attainable in man after daily oral ingestion of 600 mg, synergy of imipenem and rifampin was found for 52% of 62 P. aeruginosa and an additive effect for 37%. Against 30 S. marcescens synergy of imipenem and rifampin was not found, but an additive effect was noted for 47% of the isolates. With 32 Enterobacter isolates 35% were synergically inhibited, and an additive effect was found against 38% of the strains. Imipenem and ciprofloxacin were synergistic for 8% of P. aeruginosa and 22% of Enterobacter. Eighty-seven percent of P. aeruginosa isolates with imipenem MIC greater than or equal to 4 micrograms/ml were synergistically inhibited by the combination of imipenem-rifampin. Imipenem MIC and MBC were lowered to 1-2 micrograms/ml and to 2-4 micrograms/ml for rifampin. MIC of imipenem and ciprofloxacin were 0.5-2 and 0.05-0.1 micrograms/ml, respectively. When a triple combination of imipenem-rifampin-ciprofloxacin was studied, 62% of P. aeruginosa, 32% of Enterobacter spp. and 47% of S. marcescens were synergistically inhibited.