Skin provides good examples of various microenvironments. Skin can be compared to geographic regions of earth: the desert of the forearm, the cool woods of the scalp, and the tropical forest of the armpit. The composition of the dermal microflora varies from site to site according to the character of the microenvironment.

A different bacterial flora characterizes each of these three regions of skin:

(1) Axilla, perineum and toe webs

(2) Hand, face and trunk

(3) Upper arms and legs.

Skin sites with partial occlusion (axilla, perineum, and toe webs) are inhabitated by more microorganisms than in less occluded areas (legs, arms, and trunk). These quantitative differences may be due to increased amount of moisture, higher body temperature, and greater concentrations of skin surface lipids in these region. The axilla, perineum, and toe webs are more frequently colonized by gram-negative bacilli than are drier areas of the skin.

The number of bacteria on an individual’s skin remains relatively constant; bacterial survival and the extent of colonization probably depend partly on the exposure of skin to a particular environment and partly on the innate and species-specific bactericidal activity in skin.

The predominant resident microorganisms of the skin are:

  • Aerobic and anaerobic diphtheroid bacilli (e.g. Corynebacterium sps, Propionibacterium sps)
  • Non hemolytic aerobic and anaerobic Staphylococci (e.g. Staphylococcus epidermidis, occasionally Staphylococcus aureus and Peptostreptococcus sps);
  • Gram-positive, aerobic, spore-forming bacilli that are ubiquitous in air, water, and soil (e.g. Bacillus sps)
  • Alpha-hemolytic Streptococci (e.g. Viridans streptococci) and Enterococci (Enterococcus sps)
  • Gram-negative coliform bacilli and Acinetobacter
  • Fungi and yeasts are often present in skin folds; acid-fast, nonpathogenic mycobacteria occur in areas rich in sebaceous secretions (e.g. genitalia- Mycobacterium smegmatis)

Most microorganisms live in the superficial layers of the stratum corneum and in the upper parts of the hair follicles. Some bacteria, however, reside in the deeper areas of the hair follicles and are beyond the reach of ordinary disinfection procedures. These bacteria are a reservoir for recolonization after the surface bacteria are removed.

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