Effect of minimal amounts of thymidine on activity of trimethoprim-sulfamethoxazole against Staphylococcus epidermidis.

The antibacterial activity of trimethoprim-sulfamethoxazole against 99 strains of Staphylococcus epidermidis was tested on media known to be low in thymidine content, as determined by screening with Streptococcus faecalis. Eighty-one percent of the isolates were susceptible by agar dilution. Trimethoprim-sulfamethoxazole was bactericidal against two strains of S. epidermidis when thymidine phosphorylase was added to the medium, indicating utilization of minimal amounts of thymidine that were undetected by screening. Because bacteria vary in their utilization of thymidine and body tissues vary in thymidine content, in vitro susceptibility tests may not correlate with in vivo bactericidal activity.