In vitro susceptibility of Pseudomanas cepacia and Pseudomonas maltophilia to trimethoprim and trimethoprim-sulfamethoxazole.

Our earlier studies had shown that the two pseudomonads, Pseudomonas cepacia and Pseudomonas maltophilia, were organisms that were highly resistant to most antibiotics. The present study was undertaken to determine the susceptibility of these bacteria to trimethoprim and the trimethoprim-sulfamethoxazole combination which has been used with apparent success in treating infections caused by these pseudomonads. All 51 strains of P. cepacia were inhibited by 2 mug or less of trimethoprim per ml, whereas all 45 of the P. maltophilia were resistant to greater than 32 mug/ml. When the P. maltophilia was tested against trimethoprim-sulfamethoxazole, two strains were resistant and one only moderately resistant, but all other strains were susceptible. All P. cepacia strains were also susceptible with an average zone of inhibition significantly larger than for P. maltophilia (P < 0.005). These in vitro studies support recent case study reports of successful therapy using the trimethoprim-sulfamethoxazole combination.