Is Staphylococcus aureus part of the normal skin flora?

Staphylococcus aureus as Part of Normal Skin Flora

Staphylococcus aureus is not considered part of the normal resident flora of the skin, but rather a transient organism that can colonize the skin without causing disease in many individuals. While S. aureus commonly colonizes human skin, particularly in certain body sites, it is generally classified as a transient pathogen rather than a true commensal organism 1.

Normal Bacterial Skin Flora Classification

The bacteria on human skin are categorized into two main types:

1. Resident Flora:

   • Permanently attached to deeper layers of the skin
   • More resistant to removal
   • Less likely to be associated with infections
   • Examples: coagulase-negative staphylococci (like S. epidermidis) and diphtheroids 1

2. Transient Flora:

   • Colonize superficial layers of the skin
   • More amenable to removal by routine handwashing
   • Often acquired through direct contact with patients or contaminated surfaces
   • More frequently associated with healthcare-associated infections
   • S. aureus falls into this category 1

S. aureus Colonization Patterns

S. aureus colonization varies significantly:

• Approximately 20-30% of the general population are persistent carriers
• Another 30% are intermittent carriers
• The remainder are non-carriers
• Primary reservoirs include:
  • Nasal passages (most common)
  • Skin (particularly in moist areas)
  • Vaginal and rectal areas 1

Differences Between S. aureus and True Commensal Staphylococci

The distinction between S. aureus and true commensal staphylococci is important:

• Coagulase-negative staphylococci (like S. epidermidis) are true residents of normal skin flora 1
• S. aureus has greater pathogenic potential and is more commonly associated with skin infections 1
• S. aureus can be internalized by human skin keratinocytes and survive within them, unlike S. epidermidis 2
• S. aureus has different ecological niches compared to other staphylococci like S. lugdunensis, which is found primarily in the lower abdomen and extremities 3

Clinical Implications

Understanding S. aureus's status as a transient colonizer rather than a true resident has important clinical implications:

• S. aureus is present in only about 25% of cutaneous abscesses, despite being a common skin colonizer 1
• In atopic dermatitis, S. aureus colonization increases during disease flares, suggesting it's not part of the normal flora but rather an opportunistic pathogen 1
• Increased S. aureus abundance is associated with loss of microbial diversity in skin conditions like atopic dermatitis 1
• Unlike true commensal bacteria, S. aureus can cause severe infections ranging from superficial skin infections to life-threatening systemic disease 4

Common Misconceptions

A common misconception is that all staphylococci on the skin are part of normal flora. Important distinctions:

• Not all staphylococci should be dismissed as contaminants or normal flora when isolated from clinical specimens 5
• Coagulase-negative staphylococci are the predominant resident staphylococcal species on normal skin 1
• S. aureus has specific virulence factors that distinguish it from commensal staphylococci, allowing it to cause disease more readily 6

In conclusion, while S. aureus may colonize the skin of many healthy individuals without causing disease, it is properly classified as a transient organism with pathogenic potential rather than a true resident component of normal skin flora.