UMEM Educational Pearls

MYTH: Bactrim cannot be used as monotherapy for nonpurulent skin and soft tissue infections.

Not True!

Organisms of concern: Streptococcus spp.

Here’s why:

  • Sulfamethoxazole-Trimethoprim (Bactrim) disrupts folic acid synthesis & utilization.
  • This prevents the biosynthesis of the nucleic acid thymidine by bacteria and causes them to die. 
  • Some species of Strep, including S. pyogenes, are able to utilize exogenous sources of thymidine to continue their life cycle. 
  • Guess what? The laboratory media that was originally used to test Strep spp. susceptibility to Bactrim contained thymidine - therefore the bacteria were able to use it and did not die!
  • When tested using thymidine-depleted media, all 370 S. pyogenes isolates tested were highly susceptible to Bactrim!

TRUTH: Bactrim CAN be used as monotherapy for nonpurulent skin and soft tissue infections.

Prepared by Rianna Fedora, PharmD on 2/26/24


  1. Bowen AC et al. “Is Streptococcus pyogenes resistant or susceptible to trimethoprim-sulfamethoxazole?.” Journal of clinical microbiology vol. 50,12 (2012): 4067-72. doi:10.1128/JCM.02195-12.
  2. Coll FP et al. “Exogenous thymidine and reversal of the inhibitory effect of sulfamethoxazole-trimethoprim on streptococci.” European journal of clinical microbiology vol. 3,5 (1984): 424-6. doi:10.1007/BF02017363.
  3. McCreary EK et al. “Antibiotic Myths for the Infectious Diseases Clinician.” Clinical infectious diseases : an official publication of the Infectious Diseases Society of America vol. 77,8 (2023): 1120-1125. doi:10.1093/cid/ciad357.


SMX-TMP Myth Buster Inservice-65f3561ae6df0.pdf (58 Kb)