Comparison of the suitability of three common bacterial media for susceptibility testing of trimethoprim-sulfamethoxazole.

The in vitro inhibitory activity of trimethoprim-sulfamethoxazole is inversely related to the amount of thymine and thymidine in the test medium; manufacturers must adequately control their media to avoid spurious antimicrobial susceptibility results. No differences were observed when commercial Mueller-Hinton broth and agar were compared with a semisynthetic broth medium by the use of microdilution and disk diffusion techniques.