The skin and mucous membranes of healthy individuals maintain normal functions with naturally occurring microorganisms on them. The numbers and types of these microorganisms depend on moisture level, pH, nutrient availability, the presence or absence of inhibitory materials and the immunological tolerance of different sites of the body. Microorganisms generally found on skin, on mucous membranes and in the gastrointestinal (GI) tract of healthy individuals constitute the normal microflora, meaning they are normally present and do not cause problems in healthy individuals. People would have continual microbial infections—boils, abscesses, inflammation, diarrhea and intestinal gas/bloating—if they did not live in harmony with their normal microflora.
For more than 50 years, it has been widely believed that the presence of microorganisms on the skin is part of a natural defense because the normal microflora helps protect against pathogenic microorganisms. This dogma has been accepted without a great deal of scientific support. Billions of bacteria inhabit the GI tract. The mucous membranes of the nose, mouth and vagina may have large numbers of microorganisms. There are relatively few reported studies directed at understanding the benefits of microorganisms normally present compared with the number of studies conducted to understand mechanisms by which pathogenic microorganisms cause infections, or to determine which antimicrobial agent(s) are most effective in treatment.