What is MRSA?
MRSA (Methicillin-resistant Staphylococcus aureus) is a strain of Staphylococcus aureus bacteria that has acquired resistance to beta-lactam antibiotics through the mecA gene, which encodes an altered penicillin-binding protein (PBP2a) that allows the bacteria to continue cell wall synthesis even in the presence of methicillin and related antibiotics. 1
Mechanism of Resistance
- Resistance is conferred by the mecA gene carried on a mobile genetic element called SCCmec (Staphylococcal Cassette Chromosome mec) 1
- The mecA gene produces PBP2a, which has significantly lower affinity for beta-lactam antibiotics, allowing cell-wall biosynthesis to continue despite antibiotic presence 1
- SCCmec elements are classified into eight types (I–VIII) based on their genetic structure, with different types carrying additional resistance genes to other antibiotics like erythromycin, tobramycin, and tetracycline 1
Two Distinct Epidemiologic Forms
Healthcare-Associated MRSA (HA-MRSA)
- Traditionally acquired during hospital or healthcare facility exposure 1
- Typically carries SCCmec types I, II, III, VI, and VIII 1
- Often resistant to multiple antibiotic classes beyond beta-lactams 1
Community-Associated MRSA (CA-MRSA)
- Genetically and phenotypically distinct from HA-MRSA strains 1
- Typically carries SCCmec types IV, V, or VII 1
- May produce Panton-Valentine leukocidin (PVL) toxin, which destroys white blood cells and causes tissue necrosis 1
- Can cause rapidly progressive and devastating infections, including necrotizing pneumonia 1, 2
- Generally susceptible to a wider range of non-beta-lactam antibiotics 1
Geographic Prevalence
- Resistance rates vary significantly by region: North America (35.9%), Latin America (29.4%), and Europe (22.8%) 1
- In the United States, MRSA represents over 50% of S. aureus infections in intensive care units 3
High-Risk Populations for CA-MRSA
The following groups have increased risk for community-acquired MRSA infection 1:
- Children under 2 years old
- Contact-sport athletes
- Injection drug users
- Men who have sex with men
- Military personnel
- Inmates of correctional facilities, residential homes, or shelters
- Veterinarians, pet owners, and pig farmers
- Patients with recent influenza-like illness or severe pneumonia
- Patients with concurrent skin and soft tissue infections
- History of prior MRSA colonization or infection
- Recent antibiotic use (particularly quinolones or macrolides)
Clinical Significance
- MRSA causes a wide array of infections including bacteremia, pneumonia, skin and soft tissue infections, osteomyelitis, and septic arthritis 1, 4
- It is a leading cause of both healthcare-associated and community-associated infections worldwide 1, 5
- CA-MRSA primarily causes skin infections but can rarely cause necrotizing pneumonia with high mortality 1, 2
- MRSA pneumonia accounts for approximately 3% of community-acquired pneumonia cases, particularly in patients with prior MRSA infection or severe disease 1