Etiology

MRSA is a gram-positive pathogen that can cause skin, soft tissue, joint, and lung infections, and bacteremia. Skin and soft-tissue infections are most common, but in the US, Staphylococcus aureus is also a common cause of nosocomial pneumonia.[1]

Pathophysiology

Methicillin resistance is mediated by penicillin-binding protein 2a that is encoded by the mecA gene.[17] This allows growth of an organism in the presence of beta-lactam antibiotics. Methicillin resistance is defined as an oxacillin minimum inhibitory concentration of ≥4 micrograms/mL.[1]

Healthcare-associated and community-associated MRSA differ in several ways, including genetics and antibiotic susceptibility.

Community-associated MRSA:

  • Has different in vitro sensitivities to antimicrobials compared with healthcare-associated MRSA

  • Is more likely to be susceptible to commercially available oral antibiotics, with the exception of most beta-lactams[3]

  • Has different virulence factors

  • Has isolates that commonly contain the genes for the Panton-Valentine leukocidin toxin, a virulence factor associated with skin and soft-tissue infections. This toxin is generally not seen in healthcare-associated MRSA.

Classification

Classification according to source of infection[3]

MRSA was first recognized as a source of infection in the 1960s, with almost all people at that time having significant healthcare exposure. However, in the late 1990s, MRSA emerged as a more widespread cause of community-associated infection, with skin and soft-tissue infections being the most common.[3] This led to MRSA being categorized into 3 distinct groups as follows.

  • Healthcare-associated MRSA infection: infection occurs >48 hours into hospitalization.

  • Community-onset healthcare-associated MRSA infection: occurs outside the hospital or <48 hours into hospitalization among individuals who have had healthcare exposures in the prior year (surgery, hospitalization, hemodialysis, residence in a long-term care facility).

  • Community-associated MRSA infection: occurs in people who have not been recently hospitalized (within the last year) or have not had recent medical procedures, and may otherwise be healthy.

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