Methicillin resistant Staphylococcus aureus (MRSA) are strains of Staphylococcus aureus, or "staph," bacteria that are resistant to the antibiotic methicillin as well as to related "beta lactam" antibiotics such as oxacillin, penicillin, amoxicillin, and cephalosporins that are used to treat ordinary staph infections. These tests detect the presence of MRSA in a patient's sample.