Caution has traditionally been advised for the use of trimethoprim-sulfamethoxazole for skin and soft-tissue infections. A significant portion of these infections are caused by Group A Strep – an organism traditionally thought to be resistant to TMP-SMX.
However, these authors feel, with the rising prevalence of MRSA SSTs – for which TMP-SMX provides useful oral spectrum of activity – it is time to re-examine this dogma. Specifically, they feel the current opinion is based on inappropriate laboratory culture technique that overcomes the anti-bacterial target of TMP-SMX: thymidine metabolism. As part of ongoing clinical trials of TMP-SMX monotherapy for SSTs, they collected S. pyogenes isolates from patients and performed susceptibility testing on a variety of media they feel more appropriately reflects in vivo performance. Of the 200 isolates tested on a variety of media, only one was evaluated by Etest to be resistant to TMP-SMX. Therefore, these authors conclude TMP-SMX monotherapy may be entirely reasonable.
So, who is right? These authors – with their new culture media – or the pre-existing dogma? Unfortunately, the true answer is not yet clear – we need to look at clinical outcomes, not in vitro activity. One working theory, at least, is infected hosts seem to supply enough thymidine to enable bacteria to negate the mechanism of action for TMP-SMX in vivo. Only prospective clinical evaluation will provide better direction. I would not yet suggest TMP-SMX monotherapy for SSTs where Strep were still a possibility.
“Is Streptococcus pyogenes Resistant or Susceptible to Trimethoprim-Sulfamethoxazole?”
http://www.ncbi.nlm.nih.gov/pubmed/23052313