Tag Archives: staphylococcus aureus

“If it’s good enough for me…”

The Science of Microbiology, ,

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I do a fair bit of running, and I often think about what condition my joints will be in thirty years down the line.

Because I am a microbiologist I see first hand the devastation that can be caused by an early prosthetic joint infection (PJI) with Staphylococcus aureus.

It actually doesn’t matter whether it’s MSSA or MRSA. Either way, it’s not pretty…

For this reason, and in the knowledge that most Staphylococcus aureus PJIs are usually caused by the patient’s own colonising flora, I would be very keen to ensure that my nasal area is completely clear of Staphylococcus aureus before going under the knife.

And if it’s good enough for me, then I should definitely be considering it for my patients, and I am. The evidence is building, slowly but surely, that this is a very reasonable thing to do.

See here and here for a couple of sample papers.

There are essentially two ways to approach this:

  • Screen all patients pre-operatively for Staphyococus aureus and then give mupirocin (or another antibiotic) to those that are culture positive on screening culture.
  • Give all patients intra-nasal mupirocin prior to hip and knee joint arthroplasty surgery.

And often chlorhexidine body washes are thrown in as well to form a “bundle”.

I know there may be some institutions which are already performing Staphylococcus aureus decolonisation prior to joint replacement, but in my experience it certainly is not common practice (yet) in New Zealand. However that does not mean it is not the correct thing to do…

Obviously one would need to monitor mupirocin resistance rates, but on a population scale, giving mupirocin to a few hundred patients in a focused fashion like this does not actually represent a lot of selection pressure.

There are certainly plenty of my peers who would do exactly the same as I would when my joints wear out, even if it wasn’t “standard practice”. This is telling me something, and I hate to see “us and them” medicine practiced.

If it’s good enough for me…

Michael

“The Need for Speed”

Antimicrobial Resistance, The Art of Microbiology,

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Direct antibiotic susceptibility testing for urine samples (i.e. using the initial urine sample as the inoculum) is now fairly well established, and performed, I think, by a majority of clinical microbiology laboratories. In this way, a susceptibility report can be sent back to the requestor less than 24 hours after the sample hits the laboratory.

Direct susceptibility testing is not perfect, and you don’t need a research paper to tell you that. Common sense will tell you that if you don’t use a standardised inoculum, then occasionally you will get it wrong…

But then antibiotic susceptibility testing in general was never an exact science in the first place… (Check out this article for a bit more detail on this.)

My opinion on this is that the advantages of getting an expedited result back to the requestor far outweigh the occasional error that might occur.

With this in mind, and the advent of E-swabs, I think we should now be seriously looking at direct susceptibility testing of Staphylococcus aureus in superficial skin swabs.

I am looking at carrying out a trial setting up a Staphylococcus susceptibility plate on an inoculum taken directly from the e-swab. In this way, a Staphylococcus aureus (and MRSA) isolate could potentially be reported along with antibiotic susceptibilities in well under 24 hours.

Real time, albeit slightly imperfect microbiology….

Michael

Would be interested to know if anyone else has looked at this already.

“Open and Closed”

Antimicrobial Resistance,

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Will Staphylococcus aureus ever become completely resistant to penicillin?

At present about 10% of Staphylococcus aureus isolates worldwide retain susceptibility to penicillin. It has been like this for many years. The reason for this is probably two-fold.

  1. Bacterial populations do not exist in a closed system. Therefore they can “escape” to places where there is no selection pressure.
  2. The acquisition of resistant determinants by a bacterium almost always (if not always) comes with a fitness cost. Thus in the absence of any selection pressure, the resistant strain is unlikely to become dominant in such a hypothetical setting.

I would be very worried if Staphylococcus aureus as a population became completely resistant to penicillin, but I don’t think it will happen, not in “real time” anyway. On a population basis, I am not aware of any bacterial species which has variable resistance to an antibiotic, which has then become completely resistant to an antibiotic due to the use of that antibiotic in the population.

People worry about individual bacterial isolates being resistant to all known antibiotic classes, and label it as “The end of the antibiotic era”, which of course it is in a sense, and obviously terrible for the individual patients involved. But as microbiologists, I believe we need to be thinking about resistance much more in terms of bacterial populations, because this is where the answers to future resistance rates and the evolution of resistance really lie….

Michael