What is MRSA?
MRSA (Methicillin-resistant Staphylococcus aureus) is a strain of Staphylococcus aureus bacteria that has acquired resistance to beta-lactam antibiotics through a genetic mutation encoding an altered penicillin-binding protein (PBP2a), making it resistant to methicillin, oxacillin, flucloxacillin, cephalosporins, and carbapenems. 1
Mechanism of Resistance
The resistance mechanism is conferred by the mecA gene, which encodes PBP2a—a penicillin-binding protein with significantly lower affinity for beta-lactam antibiotics 1. This altered protein allows bacterial cell-wall biosynthesis to continue even in the presence of typically inhibitory concentrations of beta-lactam antibiotics 1. The mecA gene is carried on a mobile genetic element called SCCmec (staphylococcal cassette chromosome mec), which has been classified into eight types (I–VIII) based on genetic structure 1.
Epidemiological Categories
MRSA exists in two genetically and phenotypically distinct forms 1:
Healthcare-Associated MRSA (HA-MRSA)
- Traditionally acquired during hospital exposure or in healthcare facilities 1
- Typically carries SCCmec types I, II, III, VI, and VIII 1
- Often multidrug-resistant, carrying additional resistance genes for erythromycin (ermA), tobramycin (aadD), and tetracycline (tetK) 1
- Accounts for 64% of all S. aureus isolates in US intensive care units as of 2003 1
Community-Associated MRSA (CA-MRSA)
- Increasingly common in individuals without traditional healthcare risk factors 1
- Typically carries SCCmec types IV, V, or VII 1
- Generally susceptible to a wider range of non-beta-lactam antibiotics 1
- May produce Panton-Valentine leukocidin (PVL) toxin, which destroys white blood cells and contributes to rapid, devastating infections 1
Geographic Prevalence
Resistance rates vary significantly by region 1:
- North America: 35.9% of S. aureus isolates in skin and soft-tissue infections 1
- Latin America: 29.4% 1
- Europe: 22.8% 1
- The Netherlands: Only 0.03% due to aggressive "search-and-destroy" policies 1
High-Risk Populations for CA-MRSA
Specific populations face elevated risk for community-acquired MRSA 1:
- Children under 2 years old 1
- Contact-sport athletes 1
- Injection drug users 1
- Men who have sex with men 1
- Military personnel 1
- Inmates of correctional facilities, residential homes, or shelters 1
- Veterinarians, pet owners, and pig farmers 1
- Patients with post-influenza illness or severe pneumonia 1
- Patients with concurrent skin and soft-tissue infections 1
- History of MRSA colonization or recent infection 1
- Recent antibiotic use (particularly quinolones or macrolides) 1
Clinical Significance
MRSA causes a wide spectrum of infections including skin and soft-tissue infections, bacteremia, endocarditis, pneumonia, bone and joint infections, and central nervous system infections 1, 2. MRSA was the most common cause of skin and soft-tissue infections in US emergency departments 1. The organism can exist as asymptomatic colonization (particularly in the anterior nares) or progress to invasive infection 3. In 2005, an estimated 94,360 cases of invasive MRSA disease occurred in the United States 1.
Treatment Implications
Beta-lactam antibiotics (penicillins, cephalosporins, carbapenems) are ineffective against MRSA 1, 4. Alternative antimicrobials are required, including vancomycin, linezolid, daptomycin, tigecycline, ceftaroline, and tedizolid 1, 5. For MRSA pneumonia specifically, linezolid has shown higher clinical cure rates compared to vancomycin in prospective studies 6.