What is MRSA?
Methicillin-resistant Staphylococcus aureus (MRSA) is a strain of Staphylococcus aureus bacteria that has developed resistance to beta-lactam antibiotics, including methicillin and other commonly used penicillins and cephalosporins, making it a major public health threat that causes both healthcare-associated and community-acquired infections.
Mechanism of Resistance
MRSA achieves its antibiotic resistance through two primary mechanisms 1, 2:
- Expression of PBP2a (penicillin-binding protein 2a): This altered protein, encoded by the mecA gene, has low affinity for beta-lactam antibiotics, rendering them ineffective
- Beta-lactamase production: Enzymes that directly inactivate beta-lactam antibiotics
The mecA gene can be detected by PCR-based testing and is the molecular hallmark of MRSA 3.
Epidemiological Evolution
The epidemiology of MRSA has become increasingly complex 4, 5, 6:
- Healthcare-Associated MRSA (HA-MRSA): First identified in 1961, initially confined to hospitals and healthcare settings
- Community-Associated MRSA (CA-MRSA): Emerged in the mid-1990s among previously healthy individuals without healthcare risk factors
- Genetic distinction: CA-MRSA strains contain the SCCmec IV cassette element and Panton-Valentine leukocidin (PVL) toxin, making them genetically distinct from HA-MRSA
- Current reality: HA-MRSA and CA-MRSA strains have co-mingled in both community and healthcare settings, blurring traditional epidemiological boundaries
In some regions, up to 90% of S. aureus infections are MRSA, with infection increasing mortality risk by 64% compared to methicillin-susceptible strains 1.
Clinical Manifestations
MRSA causes a wide spectrum of infections 5, 3, 7:
Skin and Soft Tissue Infections (most common):
- Simple abscesses and boils
- Cellulitis (purulent and nonpurulent)
- Deeper infections: pyomyositis, necrotizing fasciitis
- Surgical/traumatic wound infections
Invasive Infections:
- Bacteremia (accompanies 75% of invasive MRSA cases)
- Infective endocarditis
- Pneumonia (including necrotizing pneumonia)
- Bone and joint infections (osteomyelitis, septic arthritis)
- CNS infections
- Organ abscesses (myocardial, hepatic, splenic, perinephric)
Systemic Syndromes:
- Toxic shock syndrome
- Sepsis with purpura fulminans
Treatment Challenges
CA-MRSA strains remain susceptible to many non-beta-lactam antibiotics 4, 5, with 95-100% susceptibility to agents like TMP-SMX. However, treatment options for multidrug-resistant strains remain limited.
For outpatient skin infections, oral options include clindamycin, TMP-SMX, tetracyclines (doxycycline/minocycline), and linezolid 4, 5. For hospitalized patients with complicated infections, IV vancomycin has been the traditional mainstay, though alternatives include linezolid, daptomycin, telavancin, and clindamycin 4, 5.
Public Health Impact
MRSA represents one of the most successful modern pathogens 6, causing enormous clinical and economic burden globally. Despite recent declines in incidence in some regions, MRSA continues to pose a formidable threat with persistently high morbidity and mortality 6.