9.3.2 Vancomycin-resistant S. aureus
Vancomycin resistance in S. aureus (VRSA) is an uncommon but increasing problem. It is worth learning how to test for it accurately.
Activity 12 Investigating possible VRSA in your workplace
You identify a S. aureus strain in your laboratory which you think may be resistant to vancomycin. For example, you may be concerned that a clinical isolate is resistant because the patient (human or animal depending on your laboratory) is not responding to treatment or, if it was collected as part of AMR surveillance, you might be doing routine testing for vancomycin resistance. What can you do to investigate this strain further?
Make notes in the text box and then compare with the example answer.
Discussion
You should be aware that disk diffusion methods are unreliable and not recommended for testing vancomycin resistance in Staphylococcus. It cannot accurately distinguish between wild type and resistant isolates.
In both situations described you need to find out the MIC of vancomycin for the isolate. This is easiest using a
Note: if you are aiming to treat a human patient or if the investigation is part of surveillance in animals with a One Health approach use EUCAST; if the aim is to treat an animal then vetEUCAST/vetCLSI guidelines (CLSI, 2020) should be used. EUCAST tables should tell you the breakpoint is 2 for Staphylococcus.
If the MIC of your Staphylococcus is vanA mechanism of resistance (e.g. modified cell wall as a result of exposure to the antimicrobial during treatment or via glycopeptide animal growth promoters used in farming). However, it is reported as susceptible for clinical and surveillance purposes.
If the MIC is >2 (resistant), then vanA is a possibility. In this case, the isolate should be sent to the reference laboratory for further testing which usually involves molecular detection of thevanA genes.
9.3 Confirming mechanisms in Gram-positive organisms
