Failure to demonstrate a consistent in vitro bactericidal effect of trimethoprim-sulfamethoxazole against enterococci

Controversy exists as to the in vitro and in vivo activities of trimethoprim-sulfamethoxazole (TMP-SMX) against enterococci. In this study, we investigated the in vitro activity of TMP-SMX in the type of Mueller-Hinton broth previously reported to give the lowest MICs and MBCs with enterococci. In all instances, MICs were less than or equal to 0.5 microgram/ml. The majority of tests showed MBCs of greater than 32 micrograms/ml, although there was some effect from varying the inoculum and the length of incubation after subculturing. Minor differences were noted when tests were repeated and between the results from microdilution and macrodilution tests and those obtained by the time-kill method. These results, as well as other reports, suggest that TMP-SMX should not be considered a reliable bactericidal agent against enterococci.