What is MRSA?
MRSA (methicillin-resistant Staphylococcus aureus) is a strain of staph bacteria that has developed resistance to methicillin and most related beta-lactam antibiotics, including penicillins and cephalosporins. This resistance makes MRSA infections more difficult and expensive to treat than regular staph infections.
The bacterium acquired resistance through a mobile genetic element called SCCmec (staphylococcal cassette chromosome mec), which carries the mecA gene. This gene produces a modified protein that beta-lactam antibiotics can't bind to, rendering them ineffective.
- HA-MRSA: Healthcare-associated; infections acquired in hospitals, nursing homes, dialysis centers
- CA-MRSA: Community-associated; infections in otherwise healthy people outside healthcare settings
- LA-MRSA: Livestock-associated; linked to contact with farm animals
The Evolution of Resistance
Penicillin was introduced in 1941. By 1942, penicillin-resistant staph had appeared. Methicillin was developed in 1959 specifically to combat penicillin-resistant staph. By 1961 (just two years later) the first MRSA was identified in the UK.[5]
For decades, MRSA remained primarily a hospital problem, affecting patients with surgical wounds, catheters, or weakened immune systems. Then, in the 1990s, a new pattern emerged: community-acquired MRSA (CA-MRSA) began causing severe infections in healthy young people with no healthcare exposure.
The CA-MRSA strains carried different virulence factors, including Panton-Valentine leukocidin (PVL), a toxin that destroys white blood cells and causes severe tissue damage.[2] These strains spread rapidly through sports teams, prisons, military barracks, and households.
Clinical Manifestations
MRSA causes the same infections as susceptible staph, but they're harder to treat:
- Skin and soft tissue infections: Boils, abscesses, cellulitis, impetigo
- Surgical site infections: Post-operative wound infections
- Pneumonia: Including severe necrotizing pneumonia
- Bloodstream infections: Bacteremia and sepsis
- Endocarditis: Infection of heart valves
- Osteomyelitis: Bone infections
CA-MRSA typically presents as skin infections; many patients describe being "bitten by a spider" before realizing they have a staph abscess. HA-MRSA more often causes bloodstream infections, pneumonia, and surgical site infections.
Treatment
MRSA infections require antibiotics that are still effective:[4]
- Vancomycin: IV antibiotic long considered the gold standard for serious MRSA
- Daptomycin: Alternative for bloodstream infections
- Linezolid: Oral and IV; useful for pneumonia and skin infections
- Trimethoprim-sulfamethoxazole: Oral; effective for many CA-MRSA skin infections
- Doxycycline/minocycline: Oral options for skin infections
Skin abscesses often require incision and drainage; antibiotics alone may not be sufficient. For severe infections, combination therapy may be needed.
Prevention
- Hand hygiene: The most important intervention in healthcare and community settings
- Wound care: Keep cuts and abrasions clean and covered
- Don't share: Personal items like towels, razors, athletic equipment
- Healthcare precautions: Contact isolation for infected patients
- Decolonization: Nasal mupirocin and chlorhexidine baths for carriers
Hospitals have implemented aggressive MRSA control programs including screening patients on admission, isolating carriers, and improving antibiotic stewardship. These efforts have led to significant declines in HA-MRSA infections.
The Bigger Picture: Antibiotic Resistance
MRSA is part of a larger crisis. Antibiotic resistance causes an estimated 1.27 million deaths globally each year.[1] The pipeline for new antibiotics has dried up because drug development is expensive and antibiotics are not as profitable as drugs for chronic conditions.
"MRSA was a warning shot. Without action, we face a post-antibiotic era where common infections and minor injuries can once again kill."
Vancomycin-resistant MRSA (VRSA) has already emerged, though it remains rare. The fear is that staph will acquire resistance faster than we can develop new drugs. Preserving the antibiotics we have (through stewardship and infection prevention) is critical.
Progress and Hope
There's some good news: HA-MRSA rates have declined substantially in many countries due to aggressive prevention efforts. In the US, invasive MRSA infections dropped by nearly 50% between 2005 and 2011.[3]
Research continues on new approaches:
- New antibiotics targeting different pathways
- Bacteriophages: viruses that kill bacteria
- Anti-virulence strategies that disarm bacteria without killing them
- Vaccines against S. aureus (none yet approved)
MRSA remains a formidable foe, but it's a battle that can be won with sustained investment in prevention, stewardship, and research.
Sources
- CDC. (2019). Antibiotic Resistance Threats in the United States. cdc.gov
- Tong, S. Y., et al. (2015). Staphylococcus aureus infections: epidemiology, pathophysiology, clinical manifestations, and management. Clinical Microbiology Reviews, 28(3), 603-661.
- Dantes, R., et al. (2013). National burden of invasive methicillin-resistant Staphylococcus aureus infections, United States, 2011. JAMA Internal Medicine, 173(21), 1970-1978.
- Liu, C., et al. (2011). Clinical practice guidelines by IDSA for the treatment of MRSA infections. Clinical Infectious Diseases, 52(3), e18-e55.
- Chambers, H. F., & DeLeo, F. R. (2009). Waves of resistance: Staphylococcus aureus in the antibiotic era. Nature Reviews Microbiology, 7(9), 629-641.