In vitro activity of ceftriaxone and amikacin against Pseudomonas aeruginosa

Ceftriaxone and amikacin were combined at ratios of 1:1, 5:1, and 10:1 and tested in vitro against Pseudomonas aeruginosa. The activity was determined using the Steers-Foltz replicator and the agar dilution technique with Mueller-Hinton agar. Under all conditions tested, including those simulating severe infection (10(5) to 10(6) colony-forming units per spot), the organisms were more susceptible to the combination than to the single agents. With a conventional inoculum of 10(4) colony-forming units per spot, the combinations gave 97-100% coverage against P. aeruginosa. The increased activity of the combinations resulted in MIC90 values which were below the expected serum/plasma levels for significantly longer time periods than the MIC90 values observed with the individual agents.