Methicillin‑resistant Staphylococcus aureus (MRSA) is a strain of S. aureus that resists β‑lactam antibiotics, defined by an oxacillin minimum inhibitory concentration of ≥4 µg/mL.
Explanation
MRSA emerged in the early 1960s and quickly became a leading cause of hospital‑acquired infections. Resistance results from acquisition of the mecA gene, which encodes a penicillin‑binding protein with low affinity for β‑lactam antibiotics. This gene is carried on mobile genetic elements and can be transferred among strains by bacteriophages. As a result, MRSA strains can survive treatment with methicillin, oxacillin and most other β‑lactam agents.
MRSA infections are divided into hospital‑associated (HA‑MRSA) and community‑associated (CA‑MRSA) groups. HA‑MRSA typically affects patients with risk factors such as recent hospitalization, intensive care admission, surgery, indwelling catheters or immunosuppression and often exhibits resistance to multiple antibiotic classes. CA‑MRSA occurs in otherwise healthy individuals and often causes skin and soft tissue infections but may also lead to severe pneumonia or sepsis. Both forms have substantial morbidity and mortality and require tailored therapy guided by susceptibility testing.
Clinical Significance and Risk Factors
- MRSA is a major cause of hospital‑acquired infections, including surgical site infections, ventilator‑associated pneumonia and bloodstream infections.
- Risk factors for HA‑MRSA include prolonged hospitalization, recent antibiotic exposure, invasive procedures, hemodialysis and residence in nursing homes or long‑term care facilities.
- Community‑associated MRSA often presents as abscesses or cellulitis in healthy individuals and is commonly transmitted through close contact in crowded settings.
- Methicillin resistance stems from the mecA gene, which produces a modified penicillin‑binding protein (PBP2a) that reduces affinity for β‑lactams; this gene is spread via bacteriophage‑mediated transduction.
Early recognition of MRSA and appropriate antibiotic therapy are essential for successful outcomes. Infection control practices, hand hygiene and prudent antibiotic use remain central to preventing the spread of MRSA in healthcare and community settings.
Related Terms: Staphylococcus aureus, β‑lactam antibiotics, HA‑MRSA, CA‑MRSA, mecA gene