Key Facts

MRSA is one of the most important healthcare-associated pathogens worldwide. UK programmes of mandatory screening, decolonisation, and antimicrobial stewardship have achieved dramatic reductions in MRSA bacteraemia.

Epidemiology

• UK MRSA bacteraemia: ~780 cases/year (2022) — down from >7,000/year in 2007
• MRSA accounts for ~5–10% of S. aureus bacteraemia in UK (historically >40%)
• Healthcare-associated MRSA declining; community-acquired strains increasingly recognised
• PVL-MRSA: particular concern in young, otherwise healthy individuals

Aetiology

• S. aureus carrying SCCmec (staphylococcal cassette chromosome mec) with mecA gene
• HA-MRSA: hospital-acquired, often multi-drug resistant
• CA-MRSA: community strains, often PVL-positive, may be susceptible to more agents (e.g. doxycycline, trimethoprim)
• Carriage: nasal carriage in ~2–5% of UK population; higher in healthcare workers, IVDU, prisoners

Pathophysiology

• PBP2a: altered penicillin-binding protein with low affinity for beta-lactams → resistance
• Same virulence factors as MSSA: protein A, coagulase, toxins
• PVL (Panton-Valentine leukocidin): pore-forming toxin → leucocyte lysis → tissue necrosis; associated with CA-MRSA
• Biofilm formation: important in prosthetic device infections

HA-MRSA

• Surgical site infection
• Bacteraemia (often line-related)
• Pneumonia (ventilator-associated)
• Prosthetic joint/device infection
• Wound infections

CA-MRSA (PVL-positive)

• Recurrent skin boils/abscesses (buttocks, axillae, groin)
• Necrotising pneumonia: young, previously healthy; post-influenza; haemoptysis, multi-lobar cavitating consolidation — high mortality
• Necrotising fasciitis
• Bone and joint infections

Red Flags

• S. aureus bacteraemia not responding to flucloxacillin (suspect MRSA)
• Recurrent skin abscesses (PVL-MRSA)
• Young healthy patient with cavitating pneumonia post-flu illness
• Prosthetic device with persistent infection

Screening

• Nasal swab (bilateral nares): standard MRSA screening
• Additional sites: groin, axillae, wounds, catheter sites, throat
• Enrichment culture or PCR: for rapid detection

Infection Diagnosis

• Wound swab/pus: culture and sensitivity
• Blood cultures: essential in suspected bacteraemia
• Susceptibility testing: determines antibiogram (e.g. vancomycin MIC)
• PVL gene testing: request if recurrent abscesses or necrotising infection

Bloods

• FBC, CRP, lactate: infection markers
• Vancomycin trough levels: target 15–20 mg/L for serious infections

Imaging

• Echocardiography: all S. aureus bacteraemia (TEE preferred) — endocarditis occurs in ~10–25%
• CT/MRI: metastatic infection (bone, joints, epidural abscess)
• CXR/CT chest: pneumonia

Special Tests

• Repeat blood cultures at 48–72 hours: all S. aureus bacteraemia to confirm clearance
• Removable source identification: lines, prosthetics, abscesses

Non-pharmacological

• Contact precautions: isolation in single room, gown and gloves
• Hand hygiene: soap and water (MRSA spores resistant to alcohol gel — though MRSA is not spore-forming; hand hygiene with alcohol gel is effective but soap and water preferred)
• Environmental cleaning: enhanced cleaning of patient environment
• Source removal: remove/replace infected lines, drain abscesses

Decolonisation (MRSA carriers)

• Mupirocin 2% nasal ointment TDS for 5 days (both nares)
• Chlorhexidine 4% body wash OD for 5 days
• Chlorhexidine mouthwash: if throat carriage
• Re-screen 48 hours after completion; repeat if still positive

Pharmacological

MRSA bacteraemia:

• IV vancomycin 15–20mg/kg BD (target trough 15–20 mg/L) for minimum 14 days (28 days if endocarditis)
• IV daptomycin 6–10mg/kg OD: alternative (NOT for pneumonia — inactivated by surfactant)
• Duration: minimum 14 days for uncomplicated bacteraemia; 4–6 weeks for endocarditis, osteomyelitis

MRSA skin/soft tissue:

• Oral: doxycycline 200mg then 100mg OD, trimethoprim 200mg BD, or linezolid 600mg BD (guided by sensitivities)
• IV: vancomycin, daptomycin, or linezolid
• Linezolid: monitor FBC weekly (thrombocytopenia, anaemia); max 28 days

MRSA pneumonia:

• IV vancomycin or IV linezolid 600mg BD (linezolid achieves higher lung concentrations)

Incision and drainage:

• Essential for abscesses — antibiotics alone insufficient

Referral Criteria

• Infectious diseases/microbiology: all MRSA bacteraemia
• Cardiology: echocardiography for S. aureus bacteraemia
• Surgery: abscess drainage, prosthetic device removal
• Infection prevention team: all MRSA-positive patients

• MRSA bacteraemia: mortality 20–30% (similar to MSSA bacteraemia when appropriately treated)
• PVL-necrotising pneumonia: mortality 30–75%
• Endocarditis: mortality 20–40%
• Skin/soft tissue MRSA: generally good prognosis with appropriate drainage and antibiotics
• UK MRSA bacteraemia rates reduced >80% since mandatory surveillance (2007)
• Decolonisation: effective in ~75% of carriers with single course
• Recurrent MRSA carriage: common; may need repeated decolonisation or systemic antibiotics

MRSA Treatment Summary