MRSA Skin and Soft Tissue Infections
Methicillin-resistant Staphylococcus aureus — predominant cause of purulent SSTI; treatment centers on incision and drainage plus MRSA-active oral or IV antibiotics.
Also known as: MRSA, methicillin-resistant Staphylococcus aureus, CA-MRSA, HA-MRSA
Overview
Skin and soft tissue infections caused by methicillin-resistant Staphylococcus aureus, which carries the mecA gene encoding altered penicillin-binding protein 2a (PBP2a) and is resistant to all beta-lactams except certain advanced cephalosporins (ceftaroline). Community-associated (CA-MRSA, often USA300 strain, PVL toxin-positive) differs epidemiologically and microbiologically from healthcare-associated MRSA (HA-MRSA).
Epidemiology
CA-MRSA emerged in the 2000s and now causes the majority of US purulent SSTI presenting to emergency departments. Increased among athletes, military personnel, prisoners, MSM, IV drug users, and households with close contact.
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Risk factors
- Skin trauma, prior MRSA infection, IV drug use
- Contact sports, gym/locker room exposure, sharing personal items
- Crowded living conditions (jails, military barracks, daycare)
- Recent antibiotics, healthcare exposure
- Diabetes, HIV, dialysis
- Nasal/skin colonization
Pathophysiology
S. aureus colonizes ~30% of healthy adults; MRSA colonization is the primary reservoir for autoinfection. Virulence factors include Panton-Valentine leukocidin (PVL, associated with severe SSTI and necrotizing pneumonia), alpha-hemolysin, staphylococcal protein A, biofilm formation. mecA-encoded PBP2a confers beta-lactam resistance.
Clinical presentation
Symptoms
- Painful, erythematous, indurated skin lesion with central fluctuance or pustular drainage
- Often described as 'spider bite' by patient
- Furuncles (boils), carbuncles (clusters of furuncles), abscesses, complicated cellulitis
- Fever, systemic symptoms in severe disease
- Recurrent or family-wide outbreaks suggest colonization
Signs / physical exam
- Localized warm, tender, erythematous nodule or abscess with fluctuance
- Central pustule, necrotic black-eschar in PVL-producing strains
- Lymphangitic streaking, regional adenopathy
- Systemic signs of sepsis in severe infections
Classic findings
An athlete or close-contact-setting patient with a painful, fluctuant 'spider bite' lesion — almost always purulent SSTI, most commonly MRSA in current US epidemiology.
Differential diagnosis
- MSSA SSTI — Indistinguishable clinically; identified by culture susceptibility — cefazolin/oxacillin preferred when MSSA confirmed
- Group A streptococcal infection — Often non-purulent cellulitis, lymphangitis; penicillin-susceptible
- Spider bite — Brown recluse may cause necrotizing wound — often misdiagnosed; many 'spider bites' are MRSA
- Folliculitis — Inflammatory papules around hair follicles; can progress to furuncles
- Hidradenitis suppurativa — Chronic recurrent abscesses in apocrine areas (axilla, groin, inframammary)
- Pilonidal cyst/abscess — Sacrococcygeal midline cyst, hair-bearing area
- Bartholin gland abscess — Labial swelling at posterior introitus; can be polymicrobial
Diagnostic workup
Diagnostic criteria
MRSA on culture from purulent SSTI material with confirmed resistance to oxacillin/methicillin or mecA gene detection.
Labs
- Wound culture with susceptibility testing — incise and drain first; submit purulent material
- Gram stain helpful — gram-positive cocci in clusters
- Blood cultures only for severe disease, systemic toxicity, or immunocompromised
- Routine CBC, BMP if hospitalized or systemic features
- Consider MRSA nasal swab for decolonization decisions in recurrent disease (not for acute diagnosis)
Imaging
- Bedside ultrasound distinguishes cellulitis (cobblestoning) from abscess (anechoic fluid collection)
- CT/MRI if necrotizing infection or deeper involvement suspected
Diagnostic algorithm
| Setting | Severity | Recommended Therapy |
|---|---|---|
| Outpatient — small abscess | Mild | I&D ± TMP-SMX or doxycycline x 5-7 d |
| Outpatient — large abscess or cellulitis | Moderate | I&D + TMP-SMX or doxycycline; add cephalexin if strep concern |
| Inpatient — severe SSTI/sepsis | Severe | Vancomycin IV (or linezolid, daptomycin) |
| MRSA bacteremia | Severe | Vancomycin or daptomycin; ID consult; source control |
| MRSA pneumonia (PVL-positive) | Severe | Vancomycin or linezolid; AVOID daptomycin |
| Recurrent SSTI | Outpatient | Decolonize: mupirocin nares BID x 5-10 d + chlorhexidine washes |
Treatment
First-line
- Cornerstone: incision and drainage (I&D) for any fluctuant abscess
- Outpatient MRSA-active oral antibiotics (recommended for abscesses >2 cm, surrounding cellulitis, systemic signs, immunocompromise, age extremes — per IDSA 2014):
- • TMP-SMX DS 1-2 tablets PO BID — first-line; covers MRSA but not group A strep
- • Doxycycline 100 mg PO BID — alternative; also covers MRSA, not group A strep
- • Clindamycin 300-450 mg PO QID — covers MRSA and strep; high C. difficile risk; check D-test for inducible resistance
- • Linezolid 600 mg PO BID — expensive; reserve for resistant cases; risk of serotonin syndrome and cytopenias
- If group A strep coverage also needed (non-purulent component), add cephalexin or use clindamycin alone
- Inpatient MRSA coverage (severe SSTI, sepsis, bacteremia):
- • Vancomycin 15-20 mg/kg IV q8-12h (trough/AUC targeted) — workhorse
- • Linezolid 600 mg IV/PO BID — non-inferior in MRSA pneumonia; bone marrow toxicity, MAOI interactions
- • Daptomycin 6-10 mg/kg IV daily — NOT for pneumonia (inactivated by surfactant); excellent for bacteremia/endocarditis
- • Beta-lactam for skin — cefazolin, cephalexin, dicloxacillin (only for MSSA — not MRSA)
Second-line / adjunct
- Ceftaroline — fifth-generation cephalosporin with MRSA activity; used in select cases
- Tedizolid — newer oxazolidinone with shorter course (6 days)
- Telavancin, dalbavancin, oritavancin — long-acting lipoglycopeptides; single-dose dalbavancin/oritavancin convenient for SSTI
- Decolonization for recurrent disease (≥2 episodes despite I&D): intranasal mupirocin BID × 5-10 days + chlorhexidine body washes for 5 days; consider household decolonization
Complications
- Recurrent SSTI (common — 30% within 6 months)
- Bacteremia, sepsis, endocarditis
- Necrotizing pneumonia (PVL-positive CA-MRSA — devastating, especially post-influenza)
- Osteomyelitis, septic arthritis, epidural abscess
- Toxic shock syndrome
- Necrotizing fasciitis
PANCE pearls
- 'Spider bite' presentation in the US is almost never a spider — assume CA-MRSA until proven otherwise.
- I&D is the most important intervention for abscess; many small uncomplicated abscesses (<2 cm) resolve with drainage alone, though IDSA endorses adjunctive antibiotics for most.
- TMP-SMX and doxycycline cover MRSA but NOT group A streptococcus — combine with a beta-lactam for non-purulent cellulitis where strep is the dominant concern.
- Recurrent MRSA infections in a household — decolonize the patient AND consider household-wide decolonization (mupirocin + chlorhexidine).
- Severe MRSA pneumonia — do NOT use daptomycin (inactivated by surfactant); use vancomycin or linezolid (linezolid may be preferred for toxin suppression in PVL-positive disease).
References
- IDSA 2014 — Stevens et al., Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections (Clin Infect Dis)
- IDSA 2011 — Liu et al., Clinical Practice Guidelines by the Infectious Diseases Society of America for the Treatment of MRSA Infections in Adults and Children (Clin Infect Dis)
- CDC — Methicillin-resistant Staphylococcus aureus (MRSA): Clinicians' Information
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