Tag Archives: Streptococcus pyogenes

“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…


“Impetigo: Where can the lab be of value?”

Impetigo is a common skin condition and we certainly still see plenty of it here in NZ. The word Impetigo comes from the Latin “Impetere” meaning “to attack”. It can be classified clinically into the more common non-bullous (vesicles then crusts) and the less common bullous types.

Non-bullous impetigo
Non-bullous impetigo





There are quite a few reasons why swabbing impetiginous lesions is of rather limited clinical value.

  • Most impetigo infections run their natural course. Treatment may shorten the duration of symptoms and reduce transmissability, but the fact remains that most infections will get better no matter what you do.
  • For mild to moderate impetigo the standard treatment should be with topical antiseptics or topical antibiotics (only the more severe end of the clinical spectrum requires oral antibiotics). Swabbing these lesions will not change this management in the vast majority of cases. In addition in-vitro susceptibility testing correlates very poorly with in-vivo response to topical antibiotics.
  • Impetigo is almost always caused by either Staphylocccus aureus or Streptococcus pyogenes (or both). Swabbing in order to confirm this fact is a little academic.
  • Clinical failure of treatment of impetigo often occurs in patients where the isolated bacteria have in-vitro susceptibility to the antibiotic used. I.e. there are other reasons for the treatment failure such as non-compliance, re-infection…
  • In a lot of cases impetigo is caused by the synergistic action of Staphylococcus aureus and Streptococcus pyogenes. Breaking this synergy often goes a long way to treating the condition. Therefore if both Staph aureus and Strep pyogenes are isolated from such a swab the treating antibiotic does not necessarily need to cover both bacteria to have an effect. (conceptually tricky to explain!)

So what impetigo lesions do you really need to swab? I would say if there is extensive impetigo which has not responded empirically to a reasonable course of a penicillinase resistant beta-lactam e.g. flucloxacillin (where compliance has been good), then swabbing is appropriate.

However I suspect this cohort must represent a tiny minority of clinical cases. Regardless of how big your laboratory is, if you are getting more than two or three skin swabs per week with clinical details saying “impetigo”, then you are probably getting too many….


Remember also the role of nasal swabbing (to look for Staph aureus colonisation) in patients with recurrent episodes of impetigo. This is totally appropriate!, but another story.