What does MRSA (Methicillin-resistant Staphylococcus aureus) mean?

What is MRSA?

MRSA stands for Methicillin-Resistant Staphylococcus aureus, a strain of Staphylococcus aureus bacteria that has developed resistance to methicillin and other beta-lactam antibiotics (including penicillin, oxacillin, and amoxicillin) through acquisition of the mecA gene. 1

Mechanism of Resistance

• MRSA resistance occurs through acquisition of a non-native gene (mecA) that encodes a modified penicillin-binding protein called PBP2a, which has 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 can be transferred between bacteria. 1

Two Distinct Epidemiologic Forms

Healthcare-Associated MRSA (HA-MRSA)

• Initially emerged in hospitals and healthcare facilities, first described clinically in 1961. 1
• Typically carries SCCmec types I, II, III, VI, and VIII. 1
• Often resistant to multiple non-beta-lactam antibiotics. 1

Community-Associated MRSA (CA-MRSA)

• Emerged in the mid-1990s among previously healthy individuals without healthcare risk factors. 1
• Genetically distinct from HA-MRSA, typically carrying SCCmec type IV. 1
• Often produces Panton-Valentine leukocidin (PVL) toxin, an important virulence factor that destroys white blood cells. 1
• Generally susceptible to more non-beta-lactam antibiotics compared to HA-MRSA. 1

Clinical Significance

• MRSA has become a major nosocomial pathogen globally and was responsible for an estimated 94,360 cases of invasive disease in the United States in 2005. 1
• The epidemiology has become increasingly complex as CA-MRSA and HA-MRSA strains have co-mingled in both community and healthcare settings. 1
• MRSA causes a wide spectrum of infections including skin and soft tissue infections, bacteremia, endocarditis, pneumonia, bone and joint infections, CNS infections, and toxic shock syndrome. 1

Geographic Variation in Resistance Rates

• Resistance rates vary significantly by continent: North America (35.9%), Latin America (29.4%), and Europe (22.8%). 1
• In some regions like Finland, MRSA cases are predominantly observed in elderly individuals and long-term care facilities rather than acute care hospitals. 2

Populations at Increased Risk for CA-MRSA

• Children under 2 years old 1
• Contact-sport athletes 1
• Injection drug users 1
• Military personnel 1
• Inmates of correctional facilities 1
• Patients with history of recent antibiotic use, particularly quinolones or macrolides 1