Are there other strains of Staphylococcus aureus besides Methicillin-Resistant Staphylococcus aureus (MRSA)?

Staphylococcus aureus Strains Beyond MRSA

Yes, there are multiple strains of Staphylococcus aureus beyond Methicillin-Resistant Staphylococcus aureus (MRSA), including methicillin-susceptible S. aureus (MSSA), vancomycin-intermediate S. aureus (VISA), and vancomycin-resistant S. aureus (VRSA).

Main S. aureus Strain Classifications

Based on Antibiotic Resistance

1. Methicillin-Susceptible S. aureus (MSSA)

   • Susceptible to beta-lactam antibiotics
   • Remains treatable with traditional anti-staphylococcal penicillins

2. Methicillin-Resistant S. aureus (MRSA)

   • Contains the mecA gene carried on mobile genetic elements 1
   • Produces penicillin-binding protein with reduced affinity for beta-lactams
   • Resistant to all commercially available beta-lactams and many other antistaphylococcal drugs

3. Vancomycin-Intermediate S. aureus (VISA)

   • MIC of 8-16 μg/mL for vancomycin 1
   • Reduced susceptibility to glycopeptides
   • Associated with glycopeptide treatment failure 2

4. Vancomycin-Resistant S. aureus (VRSA)

   • MIC of 32-1,024 μg/mL or more for vancomycin 1
   • High-level resistance to vancomycin
   • Rare but concerning due to limited treatment options

Based on Acquisition Setting

1. Healthcare-Associated MRSA (HA-MRSA)

   • More likely to be multi-drug resistant
   • Typically carries SCCmec types I, II, III, VI, and VIII 3
   • Different genetic background than CA-MRSA

2. Community-Acquired MRSA (CA-MRSA)

   • Generally susceptible to multiple antibiotics, often resistant only to beta-lactams 3
   • Typically carries SCCmec type IV genetic element 3, 4
   • Often produces Panton-Valentine leukocidin (PVL) toxin 4
   • USA300 clone is predominant in the United States, Europe, and Australia 4

Emerging Resistance Patterns

S. aureus has demonstrated ability to develop resistance to newer antimicrobial agents:

• Linezolid-resistant S. aureus

  • Emerging but currently rare 1, 5
  • Associated with point mutations in the 23S rRNA 5

• Daptomycin-resistant S. aureus

  • Reported in clinical isolates 2, 6
  • Complicates treatment options for serious infections

Clinical Implications

The identification of specific S. aureus strains has important implications for:

1. Treatment Selection

   • MSSA infections: First-line treatment with beta-lactams (e.g., cefazolin 7)
   • MRSA infections: Require alternative agents like vancomycin, linezolid, or daptomycin 1, 8
   • VISA/VRSA infections: May require combination therapy or newer agents 9

2. Infection Control Measures

   • Different strains may require specific infection control protocols
   • CA-MRSA transmission occurs primarily through direct skin-to-skin contact 3
   • HA-MRSA requires strict hospital infection control measures

3. Prognosis

   • MRSA infections are associated with increased morbidity, mortality risk, and healthcare costs compared to MSSA infections 6

High-Risk Populations

Certain populations have higher risk for specific S. aureus strains:

• CA-MRSA risk groups 3:

  • Children < 2 years old
  • Athletes (particularly contact-sport participants)
  • Injection drug users
  • Military personnel
  • Inmates of correctional facilities or shelters
  • Veterinarians, pet owners, and pig farmers

• HA-MRSA risk groups:

  • Hospitalized patients
  • Individuals with recent healthcare exposure
  • Those with indwelling medical devices

Prevention Strategies

• Regular hand hygiene
• Proper wound care
• Avoiding sharing of personal items
• Environmental cleaning of potentially contaminated surfaces
• Screening high-risk individuals in healthcare settings

Understanding the different strains of S. aureus is crucial for appropriate treatment selection, infection control measures, and prevention strategies to reduce morbidity and mortality associated with these infections.