Tag Archives: in vitro

“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…


“From the Laboratory to the Patient.”

Antimicrobial Resistance Testing: From the lab to the patient.

Question: What do the following have in common?

Antibiotic Dose

Tissue Penetration

Site of Infection

Patient Immunity

Pharmacokinetics of drug in patient

Extent of Infection

Wrong pathogen isolated

Polymicrobial Infection


……and probably many more.


Yes, these are all factors which dictate why even though a lab says a bacterium is resistant to an antimicrobial, the patient nevertheless gets better on that antimicrobial, and vice versa.


We are entering the murky world of in-vitro versus in-vivo response.


I often get queries from GPs wondering why MRSA infections improve on Flucloxacillin, etc etc. “How can this happen?”


The reasons are above. In the pre-antibiotic era, lots of people, particularly with minor soft tissue infections, got better without the help of antimicrobials.


Because of the myriad of different factors involved, I cannot see in-vitro susceptibility testing ever being anything more than a “best guess” at clinical response. For this reason alone, it actually makes me wonder whether we may overdo susceptibility testing and whether it should be more often reserved for the more serious infections.



MIC Breakpoints.


In-vitro susceptibility testing is based on MIC (minimum inhibitory concentration) breakpoints. Above a certain breakpoint, the bacterium is deemed resistant to the antimicrobial, and vice versa.

The MIC breakpoint is set using an ill-defined mix of epidemiological, pharmacokinetic and occasionally, clinical data.


To make matters worse, there are different organisations involved with setting standards for laboratory antimicrobial susceptibility testing. Sometimes these organisations struggle to agree on the MIC Breakpoints for different organism/antimicrobial combinations. For example look up Cefipime breakpoints for E. coli in both the CLSI and EUCAST guidelines. (What must the clinicians think of us!)


What is important to note however is that giving an MIC is better than just calling an organism susceptible or resistant. At least with an MIC value you can get an idea of the degree of in-vitro resistance or susceptibility. If the antibiotic is just reported as susceptible or resistant, then it gives the clinician no idea of how close it is to the breakpoint (from an in-vitro point of view).


I think the debate around in-vitro testing and in-vivo response will continue unabated for the rest of my lifetime…