Tag Archives: antimicrobial susceptibility testing

“Susceptibility testing for Staphylococcus aureus in community patients. Just in case..?”

Antimicrobial Resistance, Confessions of a Microbiologist, ,

You have isolated a Staphylococcus aureus from a superficial wound swab from a community patient. What antibiotics do you need to test against?

120px-Staphylococcus_aureus_(AB_Test)

If you are fortunate enough to have Automated Susceptibility testing (Vitek, Phoenix etc) you may test the isolate against a panel of 20 different Gram positive antibiotics at a not unsignificant cost. You will also be testing against a lot of antibiotics that are never going to be used to treat that patient.

If you perform anti-microbial susceptibility testing mainly by disc diffusion on agar plate, then you will likely test the Staphylococcus aureus against 6 or 7 antibiotics of your choosing. Again, in the vast majority of patients, some of these antibiotics will neither be reported nor used.

In the laboratory I work in, we cut the number of antibiotics routinely tested for a Staphylococcus aureus isolate from six to three, of which we routinely report two of them to the clinicians. In the three years that we have had this policy we have not had one negative comment from our laboratory users. (On occasion of course we do test more, ie MRSA isolate, patient with multiple allergies.)

It made me realise just how much susceptibility testing that we do on a “Just in Case” basis, not only for Staphylococcus aureus, but also for a multitude of other micr-organisms also.

It will be very interesting if MALDI-TOF technology becomes proficient at identifying MRSA in the future. If this happens, then (if no history of allergy on the request form), one could simply put out a report saying: “Staphylococcus aureus isolated, (not MRSA). Susceptibilities on request.”

A radical, but ultimately reasonable and cost-effective approach, which I am sure would be accepted by laboratory users…

Michael

“Antimicrobial Susceptibility Testing for Chlamydia: Out of Sight, out of mind…”

The Science of Microbiology, ,

We are fortunate with Chlamydia trachomatis, in that the organism so far, retains almost universal susceptibity to a range of oral antimicrobials, including tetracyclines, erythromycin, azithromycin and even amoxycillin.

This is just as well really as Chlamydia trachomatis is almost exclusively diagnosed by molecular means and antimicrobial susceptibility testing is both difficult and non-standardised.

What we don’t really know about Chlamydia trachomatis is whether continued heavy usage of tetracyclines and azithromycin are leading to “MIC creep” to these antimicrobials. It may be that we only start worrying about this when we start to see treatment failures in significant numbers.

The worry would be that as now Neisseria gonorrhoeae moves towards molecular diagnosis, we will apply the same “out of sight, out of mind” philosophy to this micro-organism. However we definitely cannot afford to go down this route for Neisseria gonorrhoeae as there are already significant resistance problems with this particular bacterium.

We need to remember for both C. trachomatis and N.gonorrhoeae, that just because we aren’t doing susceptibility testing, it doesn’t mean these micro-organisms have suddenly stopped selecting out resistant mutants under antimicrobial pressure….

Michael

p.s. While on the subject of GU microbiology, here is a really nice article on the conditions of cytolytic vagnosis and lactobacillosis, well worth a read.