Definition:

Staphylococcus aureus is a Gram-positive human commensal bacterium persistently colonizing the anterior nares of about 30% of the human population. Methicillin-resistant S. aureus (MRSA) is known to have evolved from methicillin-susceptible S. aureus (MSSA) after acquiring the staphylococcal cassette chromosome mec (SCCmec) element which includes the mecA gene coding for resistance to the antibiotic methicillin. A highly cytotoxic and clinically virulent methicillin-sensitive strain, S. aureus strain 6850 is a well-characterized prototype strain isolated from a patient with a skin abscess which had progressed to S. aureus bacteremia, osteomyelitis, septic arthritis, and multiple systemic abscesses.