Co-trimoxazole susceptibility tests improved with separate trimethoprim and sulfamethoxazole disks.

It is impossible to test accurately bacterial susceptibility to the trimethoprim-sulfamethoxazole combination co-trimoxazole with a single combined susceptibility disk. However, a variety of factors still affect the result even when separate trimethoprim and sulfamethoxazole disks are used. Experiments with separate disks showed that the optimum conditions for testing the susceptibilities of enterobacteria to these drugs were to flood-seed an agar plate with an inoculum of 10(4) to 10(5) organisms per ml, take off the excess liquid, and place a disk of 1 microgram of trimethoprim and another of 50 micrograms of sulfamethoxazole on the surface of the agar with their centers exactly 25 mm apart. This method not only allowed the determination of resistance but also distinguished synergy.