Methicillin-Sensitive Staphylococcus aureus (MSSA)
MSSA is a strain of Staphylococcus aureus bacteria that remains susceptible to methicillin and other beta-lactam antibiotics, causing a wide range of infections from skin and soft tissue infections to more severe conditions like bacteremia, endocarditis, pneumonia, and bone/joint infections. Unlike its counterpart MRSA (Methicillin-Resistant Staphylococcus aureus), MSSA can be effectively treated with beta-lactam antibiotics such as nafcillin, oxacillin, and cephalosporins.
Characteristics and Significance
- MSSA is part of the normal skin flora in many individuals, primarily colonizing the anterior nares, axillae, and perineum
- Despite being susceptible to methicillin, MSSA can cause the same spectrum of infections as MRSA 1
- While historically considered a community pathogen, recent evidence suggests MSSA may now be more commonly associated with healthcare settings than MRSA in some regions - a potential role reversal 2
Common MSSA Infections
- Skin and soft tissue infections (including impetigo)
- Bacteremia and endocarditis
- Pneumonia
- Bone and joint infections
- Central nervous system infections
Antibiotic Treatment for MSSA Infections
Beta-lactam antibiotics are the preferred treatment for MSSA infections due to their superior efficacy:
First-line treatments:
- Antistaphylococcal penicillins: Nafcillin or oxacillin are the drugs of choice for serious MSSA infections 3
- First-generation cephalosporins: Cefazolin is commonly used as an alternative to nafcillin due to better tolerability and cost 3
For beta-lactam allergic patients:
- Vancomycin: Can be used but has been associated with poorer outcomes compared to beta-lactams for MSSA infections 3
- Daptomycin: Reasonable alternative for patients with beta-lactam allergies 3
Treatment duration:
- For uncomplicated left-sided native valve endocarditis: 6 weeks of nafcillin or equivalent antistaphylococcal penicillin 3
- For complicated left-sided native valve endocarditis: At least 6 weeks of nafcillin or equivalent antistaphylococcal penicillin 3
Important Clinical Considerations
- Beta-lactam allergy evaluation: Should be conducted in every case where vancomycin is considered for MSSA treatment due to poorer outcomes with vancomycin 3
- Brain abscess: Nafcillin should be used instead of cefazolin for MSSA brain abscesses; vancomycin should be given in cases of nafcillin intolerance 3
- Avoid gentamicin: Gentamicin should not be used for treatment of native valve endocarditis caused by MSSA 3
- Clindamycin caution: Has been associated with IE relapse and is not recommended for endocarditis 3
Distinguishing from MRSA
- MSSA differs from MRSA primarily in its susceptibility to beta-lactam antibiotics
- Both can cause identical clinical syndromes with similar virulence 4
- Proper identification through culture and susceptibility testing is crucial for optimal treatment
- Empiric therapy may be necessary before culture results are available, especially in severe infections
Antimicrobial Resistance Patterns
- MSSA is widely resistant to penicillin due to beta-lactamase production 3
- Approximately 10-14% of MSSA isolates show resistance to macrolides 3
- Only 2-8% of MSSA isolates demonstrate tetracycline resistance 3
- Modern fluoroquinolones (levofloxacin, moxifloxacin) generally maintain activity against MSSA 3
Understanding the distinction between MSSA and MRSA is critical for appropriate antibiotic selection and optimal patient outcomes. While both organisms can cause similar infections, the treatment approaches differ significantly based on susceptibility patterns.