Tag Archives: thymidine content

“On second thoughts…”

I love fallacies, old wives tales, and urban myths…

You are probably familiar with the dogma that you should never use cotrimoxazole to treat infections due to Streptococcus pyogenes.

In the laboratory setting we have traditionally never been much good at in-vitro susceptibility testing for cotrimoxazole against Streptococcus pyogenes…


In vitro susceptibility testing of Streptococcus pyogenes against cotrimoxazole is dependent on/vulnerable to the amount of thymidine in the susceptibility media. Thymidine allows Streptococcus pyogenes to bypass sulphonamide mediated inhibition of folate metabolism.

In the past, media contained unregulated (and often high) levels of thymidine, particularly those media that contained blood. As a consequence Streptococcus pyogenes survived quite happily on such media, even in the presence of sulphonamides and thus showed in-vitro resistance.

However modern media such as Mueller-Hinton agar (MHA) are now regulated as to their thymidine content. A study in the Journal of Clinical Microbiology in 2012 using such agar showed at least 99% in-vitro susceptibility of Streptococcus pyogenes to cotrimoxazole.

It is likely that our poor laboratory practice in the past has led to cotrimoxazole being labelled a ”No-No” for infections due to Streptococcus pyogenes.

However this information above only refers to in-vitro susceptibility testing. Whether this translates into in-vivo susceptibility/clinical response is another question altogether, and in the modern day, will likely  need clinical trials to answer. (See this article for a bit about in-vitro and in-vivo susceptibility)

All the textbooks in my lab say that Streptococcus pyogenes is resistant to cotrimoxazole. That’s because they are all 10-20 years old!

It is difficult to change people’s minds at the best of times. Old habits die hard…