Deciphering the mechanism of mecA positive Oxacillin sensitive MRSA

My work is on finding how this particular type of MRSA manages to switch it's phenotype from Oxacillin sensitive to resistant when its given an antibiotic exposure at sub inhibitory concentration or when given in vivo. Because even though, it harbours a mecA gene during routine clinical MIC tests the results come as oxacillin sensitive (or as an MSSA) but when the patient is given MSSA treatment the drugs show no effect and bacteria showcase a MRSA phenotype. My job is to find out how !