Tissues and exudates contain sufficient thymidine for growth of anaerobic bacteria in the presence of inhibitory levels of trimethoprim-sulfamethoxazole.

Obligate anaerobes are susceptible in vitro to trimethoprim-sulfonamides. Clinical efficacy of this drug combination for treatment of infectious processes in which anaerobes are involved is uncertain. We hypothesize that this uncertainty is a result of thymidine in tissues and exudates which abrogates the inhibitory effect of trimethoprim-sulfonamides. We shown herein that species of anaerobic bacteria commonly encountered in infectious processes grew on thymidine-containing medium despite the presence of levels of trimethoprim-sulfamethoxazole (S x T) previously shown to be inhibitory. Biologic fluids and tissues, where anaerobic bacteria are commonly encountered in diseased states, were shown to contain thymidine levels that were sufficient to reverse the inhibitory effect of S x T upon these species of bacteria. These observations suggest that the use of trimethoprim-sulfonamides is not a rational choice for treatment of infectious processes in which an obligate anaerobe is a component.