Trimethoprim-resistant bacteria: cross-resistance patterns.

One hundred and eighty one strains (clinically isolated) of trimethoprim-resistant bacteria were tested for sensitivity to proguanil, pyrimethamine, methotrexate and methasquin. All these agents had less intrinsic antibacterial activity than trimethoprim, and there was marked cross-resistance between trimethoprim on the one hand and pyrimethamine and methasquin on the other. Methotrexate was of very low activity against Gram-negative bacilli, but was highly inhibitory for streptococci. Proguanil had approximately the same activity for all the organisms tested, and appears not to act as an anti-folate agent.