Is Staphylococcus aureus part of normal skin flora?

Staphylococcus aureus is NOT Part of Normal Skin Flora

Staphylococcus aureus is not considered part of the normal skin flora, but rather a pathogen that can transiently colonize the skin and cause infections when it breaches the skin barrier. While coagulase-negative staphylococci (such as S. epidermidis) are normal resident flora of the skin, S. aureus is a pathogen that commonly causes skin and soft tissue infections.

Normal Skin Flora Composition

The normal bacterial skin flora consists primarily of:

• Resident flora - These bacteria are attached to deeper layers of the skin and are more resistant to removal 1. They include:

  • Coagulase-negative staphylococci (particularly S. epidermidis)
  • Diphtheroids
  • Other non-pathogenic gram-positive bacteria

• Transient flora - These bacteria colonize the superficial layers of the skin and are more easily removed by routine handwashing 1. They are often acquired through:

  • Direct contact with patients
  • Contact with contaminated environmental surfaces
  • These organisms are most frequently associated with healthcare-associated infections

S. aureus as a Pathogen, Not Normal Flora

S. aureus is characterized as a pathogen rather than normal flora for several key reasons:

• It is specifically identified as a pathogen in multiple skin infections including:

  • Impetigo and ecthyma 2
  • Folliculitis, furunculosis, and carbuncles 1, 2
  • Staphylococcal scalded skin syndrome 2
  • Cellulitis and abscesses 1

• Guidelines specifically note that S. aureus is present in only about 25% of cutaneous abscesses, usually as a single pathogen, indicating its role as an invader rather than normal flora 1

• S. aureus can be transiently carried in the nose before causing cutaneous disease, suggesting it's not a normal resident of the skin itself 1

• Unlike normal flora, S. aureus possesses virulence factors that allow it to cause disease when it breaches the skin barrier 1, 3

Distinguishing S. aureus from Normal Skin Flora

The distinction between S. aureus and normal skin flora (like coagulase-negative staphylococci) is important for several reasons:

1. Pathogenicity profile: S. aureus possesses virulence factors not found in normal flora like coagulase-negative staphylococci 1

2. Antibiotic resistance concerns: S. aureus, particularly MRSA strains, requires specific antibiotic approaches 1

3. Clinical significance: When S. aureus is cultured from skin infections, it should be considered a pathogen requiring treatment, while coagulase-negative staphylococci may sometimes be contaminants 4

Clinical Implications

Understanding that S. aureus is not normal flora but a pathogen has important clinical implications:

• Presence of S. aureus in wound cultures should be treated as potentially pathogenic, not dismissed as normal flora
• Infection control measures should focus on preventing S. aureus transmission
• Patients with recurrent S. aureus infections may benefit from decolonization strategies, as S. aureus can persist within keratinocytes and evade antibiotic killing 5

Common Pitfalls

1. Misinterpreting culture results: Coagulase-negative staphylococci are often normal flora, but S. aureus should be considered a pathogen when isolated from clinical specimens

2. Inadequate treatment: S. aureus can internalize within keratinocytes, evading many standard antibiotics 5

3. Confusing colonization with normal flora: While S. aureus can colonize the skin (especially in healthcare settings), this represents transient colonization, not normal flora status

In conclusion, while the skin is naturally colonized by various bacteria, S. aureus is properly classified as a pathogen that can transiently colonize the skin rather than a component of normal skin flora.