Don’t mess with Staph Aureus

Imagine you are an internal medicine resident on call overnight in the hospital. You admit a patient with high fevers of unclear etiology. The patient had blood cultures drawn in the emergency room (before antibiotics, thank goodness) and is started on empiric vancomycin and cefepime. The next day, the micro lab calls you and says that her blood cultures are growing “Staphylococcus Aureus.”

You recall from medical school that Staph Aureus is divided into two groups, the feared “MRSA” and the less-feared “MSSA.” But at this time you don’t know if the patient has MRSA or MSSA. It’s a good thing you have your patient on vancomycin in case the bacteremia is due to MRSA. But is your patient on the correct treatment?

You may not know what’s going on behind the scenes, in the micro lab, and that can help you manage her infection.

After the blood cultures turn positive, a gram stain is performed and the isolate is plated on the appropriate media. Direct non-standardized Kirby-Bauer plates are then performed which can give you a preliminary “hint” if the patient has MRSA or MSSA. If the plate has a small zone of inhibition around “FOX” (oxacillin), and P (PCN), that suggests MRSA (see photo below). If there is a large zone of inhibition, that suggests MSSA.

I suggest you phone the micro lab at this time, and ask for the “blood room” and request that the technician look at the Kirby-Bauer plate. If the isolate looks like it’s likely going to be MSSA, you should continue the vancomycin but add nafcillin (and drop the cefepime). Nafcillin is better than vancomycin or cefepime for MSSA. If the isolate looks like it is going to be MRSA, you should continue the vancomycin (and drop the cefepime). And get 2 more sets of blood cultures.

The next day you will have your official sensitivities in the computer with your quantitative MICs, and you will know if the Kirby-Bauer guess was correct. If it’s MSSA, you can drop the vancomycin and continue nafcillin monotherapy. If it’s MRSA, you can continue vancomycin monotherapy.

Meanwhile, I hope you are looking hard for the source of the bacteremia and are thinking about where staph aureus may have gone (heart valves, bones, joints, abscesses, etc). Get an infectious diseases consultation to help you. If there’s anything I’ve learned in my ID fellowship, it’s “don’t mess with Staph Aureus.”

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