Question
Normal flora of skin
Answer
The normal flora of the skin comprises microorganisms that permanently or transiently colonize the skin surface without normally causing disease, playing an important protective role by competing with and inhibiting potential pathogens (colonization resistance).
Resident (permanent) flora:
- Staphylococcus epidermidis (coagulase-negative staphylococcus) — the most abundant resident organism; important opportunistic pathogen in the context of indwelling medical devices (biofilm formation).
- Corynebacterium species (diphtheroids) — commonly found in moist areas (axilla, groin).
- Propionibacterium acnes (now Cutibacterium acnes) — an anaerobic organism residing in sebaceous glands/hair follicles, implicated in the pathogenesis of acne vulgaris.
- Malassezia species (lipophilic yeasts) — normally commensal, but can become pathogenic (causing pityriasis/tinea versicolor, and implicated in seborrhoeic dermatitis) under favourable conditions (heat, humidity, oiliness).
- Micrococci and various coagulase-negative staphylococci.
Transient flora: organisms acquired temporarily from the environment or other body sites (e.g., Staphylococcus aureus, Gram-negative bacilli), which do not permanently establish but can persist for hours to days, particularly relevant to hand hygiene and infection control (transient flora is the primary target of routine handwashing).
Clinical significance: resident skin flora is generally protective (via competitive exclusion, production of inhibitory substances, and maintenance of an acidic skin pH unfavourable to many pathogens) but can become opportunistically pathogenic when skin integrity is breached (wounds, catheters, surgery) or in immunocompromised hosts — S. epidermidis is a leading cause of catheter/prosthetic-device-associated infection precisely because it is so abundant on normal skin, allowing easy introduction during device insertion.