Methicillin-resistant Staphylococcus aureus (MRSA) is a Gram-positive bacteria responsible for several difficult-to-treat infections in humans. MRSA is any strain of Staphylococcus aureus that has developed resistance to beta-lactam antibiotics, which include the penicillins and the cephalosporins. Strains susceptible to these antibiotics are classified as methicillin-sensitive Staphylococcus aureus, or MSSA.
MRSA is the etiologic pathogen for the majority of health care‚Äďassociated infections, and it creates a huge burden on the health care system. In US hospitals, more than 50% of the S. aureus isolates were MRSA and nosocomial MRSA infection is associated with much higher morbidity, mortality, and medical costs than infection caused by MSSA.