By Normal flora, we mean the population of microorganisms residing in  the skin and mucous membranes of  healthy normal individuals without causing any harm to them. There are more numbers of bacteria (1014) in human body than the number of cells (1013). These bacterial populations constitute the normal microbial flora of human body. The normal flora is relatively stable, with specific bacteria colonizing various body regions during particular periods in an individual’s life. They are acquired by every human being from shortly after birth until death. Viruses and parasites are not considered members of the normal microbial flora because they are not commensals and harm the host. 

The skin and mucous membranes of an individual is always colonized by a variety of microorganisms. These microorganisms can be arranged into two groups:

  • Resident flora
    • It consists of relatively fixed types of microorganisms regularly found in a given area at a given age and if disturbed, it promptly reestablishes itself
  • Transient flora
    • It consists of nonpathogenic or potentially pathogenic microorganisms that inhabit the skin or mucous membranes for hours, days, or weeks. It is derived from the environment, does not produce disease, and does not establish itself permanently on the surface. Transient flora is generally of little significance as long as the normal resident flora remains intact. However, if the resident flora is disturbed, transient microorganisms may colonize, proliferate, and produce disease.

Roles of Normal Flora

Normal flora in the intestinal tract synthesizes vitamin K and help in the absorption of nutrients. On mucous membranes and skin, normal flora may prevent colonization by pathogens either by competition for receptors or binding sites on host cells, competition for nutrients, mutual inhibition by metabolic or toxic products, mutual inhibition by antibiotic materials or bacteriocins, or other mechanisms. Suppression of the normal flora clearly creates an opportunity for the microorganisms from the environment or from other parts of the body to colonize these sites. Such organisms may behave as opportunists and may become pathogens.

On the other hand, members of the normal flora may themselves produce disease under certain circumstances. If these organisms reach a sterile site, they may become pathogenic. For example, if Escherichia coli that is a normal intestinal flora reaches urinary tract, it may cause urinary tract infection.

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