Exogenous thymidine and reversal of the inhibitory effect of sulfamethoxazole-trimethoprim on streptococci.

The practice of using sulfamethoxazole-trimethoprim (SXT) for the selective isolation of Streptococcus pyogenes and as a taxonomic character in the presumptive identification of streptococci was applied to 17 strains of different groups of streptococci to determine their characteristic behaviour in the presence of exogenous thymidine. Streptococcus pyogenes, Streptococcus agalactiae and group D enterococci utilized thymidine, the first two species obtaining a maximum reversal of the inhibitory effect of SXT at thymidine concentrations of 1.2 micrograms/ml and 0.6 micrograms/ml or higher, respectively. For group D enterococci, the degree of reversal of the inhibitory effect was proportional to the thymidine concentration. In contrast, the four viridans species studied (Streptococcus sanguis I, Streptococcus salivarius, Streptococcus mitis and Streptococcus sanguis II) and Streptococcus pneumoniae were unable to utilize thymidine from an exogenous source and thus growth remained inhibited even at the highest concentrations of thymidine tested. For selective isolation and identification of streptococci only stable media with batch-to-batch consistency are recommended together with a known quantity of thymidine.