methicillin-resistant Staphylococcus aureus (MRSA)

Etiology

• unnecessary antibiotic use in general
• antibiotics in food
• bacterial mutation
• risk factors for nosocomial infection:
  • current or recent hospitalization treated with parenteral antibiotics
  • residing in a long-term care facility
  • invasive procedures
  • recent or long-term antibiotic use
  • prior infection with influenza ?
  • prior respiratory isolation of MRSA
  • critically ill
• risk factors for community-acquired infection
  • young age - incomplete development of immune system
  • participation in contact sports
  • sharing towels or athletic equipment
  • immunosuppression (AIDS)
  • incarceration, crowded or unsanitary conditions
  • end-stage renal disease
  • injection drug use
• compared with MRSA carriers, risk of MRSA infection are 14 times lower for uncolonized patients & 125 times lower for MSSA carriers

Epidemiology

• healthcare-associated infection (80%)
• nursing homes
  • SCCmec II strains (hospital associated) 73% SCCmec IV strains (community-acquired) 8%
  • foley catheter associated with groin or perianal colonization with MRSA
• outpatients
  • 60% of community-acquired Staphylococcus in parts of Alaska, California, Georgia & Texas)
  • most outpatient infections involve skin & soft tissue
  • 70% of staphylococcal skin infections due to MRSA
  • healthcare-associated infection accounts for 76% of outpatient MRSA
• inpatients:
  • 51% of MRSA+ patients at discharge clear MRSA within 1 year
  • mean time to clearance 246 days
• asymptomatic carriers
  • risk factors
  • smokers
  • insulin-requiring diabetes mellitus
  • hemodialysis
  • injection drug users
• transmission is by contact
• 17% overall decrease in community-onset & a 28% overall decrease in hospital-onset healthcare-associated infections between 2005 & 2008
• decrease in MRSA between 2005 & 2011
  • invasive nosocomial MRSA: 54%
  • healthcare-associated community-onset MRSA: 28%

Pathology

• skin infections & soft tissue infections (most common)
• pneumonia
• resistance due to alterations in penicillin-binding protein(s)
• virulence may be related to Panton-Valentine leukocidin

Genetics

• 99% of isolates from single clone 'USA 300' 1% of isolates from 'USA 400'
• CA strains ?

Laboratory

• D-test identifies erythromycin resistance with inducible resistance to clindamycin
• PCR provides rapid testing for MRSA
  • methicillin resistance gene
  • MRSA nucleic acid
    • MRSA DNA
  • Staphylococcus aureus + MRSA nucleic acid
• MRSA identified in isolate
• methicillin resistant Staphylococcus aureus culture
• blood cultures for bacteremia, sepsis
  • repeat every 2-4 days until negative
  • median time to clearance of MRSA is 7-9 days

Diagnostic procedures

• transthoracic echocardiogram (TTE)
• transesophageal echocardiogram if TTE negative

Radiology

• CT of abdomen & pelvis if transesophageal echocardiogram negative
• MRI of spine rule out/in spinal epidural abscess if back pain
• FDG PET-CT if no diagnosis
• tagged leukocyte nuclear scan a late option

Complications

• long-term carrier state in 20% of patients

• disease interaction(s) of bacteremia with MRSA in patients with prosthetic joint
• disease interaction(s) of influenza with MRSA

Management

• abscess
  • incision & drainage
• pharmaceutical agents
  • consider resistant to all beta-lactam antibiotics
  • may also be resistant to macrolides, fluoroquinolones, Bactrim, clindamycin, aminoglycosides
  • nosocomial infection or patients requiring hospitalization:
    • multidrug-resistance common
    • IV vancomycin or daptomycin for MRSA sepsis
      • therapy for >= 14 days
      • 4-6 weeks of therapy for MRSA sepsis in a patient with arthroplasty
      • median time to clearance of MRSA bacteremia is 7-9 days
      • adding beta-lactam to vancomycin or daptomycin for MRSA sepsis of no benefit
    • linezolid useful as oral agent
      • may have advantage over vancomycin
      • not indicated for treatment of MRSA endocarditis or sepsis
    • trimethoprim/sulfamethoxazole plus rifampicin noninferior to linezolid
    • trimethoprim/sulfamethoxazole inferior to vancomycin especially for sepsis
    • ceftaroline (5th generation cephalosporin) IV
  • ouptatient: (skin infections)
    • Bactrim, doxycycline, minocycline, or clindamycin
    • 2 Bactrim DS or Septra DS PO BID-TID
    • clindamycin may diminish duration of carrier state after resolution of index infection relative to trimethoprim/sulfamethoxazole
  • vancomycin resistance; MIC > 2 ug/mL
    • daptomycin 10 mg/kg/day plus a second agent such as gentamicin, rifampin, linezolid, TMP-SMX, or a beta-lactam antibiotic
    • do not use daptomycin for staphylococcal pneumonia (inactivated by surfactant)
    • quinupristin-dalfopristin, TMP-SMX, linezolid, or telavancin alone or in combination for daptomycin resistance
    • fosfomycin has activity against MRSA & VRE
• carrier state
  • eradication of carrier state with mupirocin (Bactroban) to anterior nares (of nose), fingernails & to wounds
    • twice-monthly decolonization with mupirocin for 6 months reduces post hospital discharge MRSA infections
    • apparently MRSA resistance develops
  • chlorhexidine (Hibiclens) baths
  • mupirocin, chlorhexidine, & bleach-water can temporarily eradicate Staphylococcus aureus colonization
  • do NOT treat carrier state with systemic agents
  • hot coffee &/or tea reduces risk of MRSA nasal carriage by ~ 50%
• bleach baths may be of benefit for recurrent Staphyloccus infection (MRSA or MSSA)
• infection control; contact precautions
  • handwashing & barrier techniques to prevent transmission
    • alcohol-based hand sanitizer effective
  • maintain contact precautions in patients not receiving an antibiotic active against MRSA until 1-3 negative cultures obtained
  • isolation of patients with MRSA to prevent outbreaks
  • isolation precautions not needed in communities where MRSA is endemic

More general terms

• Staphylococcus aureus

Additional terms

• nosocomial infection; nursing home acquired infection
• penicillin binding protein