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If a Staph aureus isolate is a high beta-lactamase producer and not a MRSA, can you get growth on the OX screen plate but have a Susceptible result for Oxacillin?
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We have seen rare isolates of Staphylococcus aureus that grow on oxacillin screen agar, but are susceptible to oxacillin by either broth microdilution or disk diffusion. These isolates are negative for the PBP2a-encoding gene mecA. The mechanism for reduced susceptibility to oxacillin (i.e. growth on the screen agar) is unknown. It could be hyper beta-lactamase production or it could be changes in native PBP or an unknown mechanism.
When this is seen in the lab, it would be good to confirm the isolates identification and purity. If these are okay, then the result from the oxacillin MIC or disk diffusion susceptibility test should be reported rather than the result of the oxacillin screen agar.
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