Microbiologist holding up two plates of methicillin-resistant Staphylococcus aureus (MRSA). MRSA is a bacterium responsible for several difficult-to-treat infections in humans. It is also called oxacillin-resistant Staphylococcus aureus (ORSA). MRSA is any strain of Staphylococcus aureus that has developed, through the process of natural selection, resistance to beta-lactam antibiotics, which include the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins. Strains unable to resist these antibiotics are classified as methicillin-sensitive Staphylococcus aureus, or MSSA. According to the Centers for Disease Control and Prevention, more than two million people in the United States get infections that are resistant to antibiotics and at least 23,000 people die as a result. Over-prescription of antibiotics leads to antibiotic-resistant germs.