Susceptibility of Pseudomonas maltophilia to antimicrobial agents, singly and in combination.

Pseudomonas maltophilia is resistant to most of the commonly used antimicrobial agents including those active against Pseudomonas aeruginosa. The susceptibility of 14 clinical isolates of P. Maltophilia to 18 antimicrobial agents was determined by broth dilution testing. All organisms were susceptible to trimethoprim-sulfamethoxazole (TMP-SMZ), minocycline, and LY127935. A total of 87 and 79% of the organisms were susceptible in vitro to colistin and chloramphenicol, respectively. With the exception of sisomicin, the organisms were resistant to the aminoglycosides. Of 21 combinations of antimicrobials examined for synergy, only the combination of TMP-SMZ with carbenicillin was consistently (86%) synergistic in vitro. Supplementation of the testing media with calcium and magnesium increased the minimal inhibitory concentrations for the aminoglycosides, the penicillins, and TMP-SMZ against P. maltophilia.