Synergistic activity of amikacin with aztreonam against Pseudomonas aeruginosa and other gram-negative organims

The effects of amikacin in combination with aztreonam were tested on 100 strains of gram-negative bacteria by means of the microtiter checkerboard system. Eighty-one strains were Pseudomonas aeruginosa, eight were Enterobacter species, seven were Acinetobacter species, two were Alcaligenes species, one was a Citrobacter organism, and one was Pseudomonas stutzeri. All strains had a minimal inhibitory concentration (MIC) to aztreonam between 4 and 256 μg/ml (MIC₅₀ = 16 μg/ml) and an MIC to amikacin between 8 and > 128 μg/ml (MIC₅₀ = 16 μg/ml). A synergistic effect (fourfold less MIC when both aztreonam and amikacin were present) was observed for 40 (49%) of 81 P aeruginosa and three (16%) of 19 of the other strains tested. All the remaining test strains showed an additive response; no antagonism was observed. The MICs to aztreonam and to amikacin for 98% of strains that exhibited synergism ranged from 2 to 32 μg/ml. These results suggest that amikacin and aztreonam may be a synergistic antimicrobial combination against some gram-negative organisms, including P aeruginosa. Synergy was more likely with those strains that had MICs within achievable blood levels for both agents.