Normal Microbial Flora
The term “normal microbial flora” denotes the population of microorganisms that inhabit the skin and mucous mem-brane of normal healthy individuals. It has been estimated that humans have approximately 1013 cells in their body and about 1014 bacteria are associated with them. The majority of bacteria are present in the large bowel, which constitutes the normal flora. The organisms are present in those parts of the body that are exposed to, or communicate with, the external environment, namely, the skin, nose, mouth, and intestinal and urogenital tracts. Internal organs and tissues are normally sterile.
The human fetus, in pregnant mother, lives in a sterile environment protected from microbes except when pathogens like cytomegalovirus, rubella virus, or Toxoplasma gondii cross the placenta for first 9 months of life. At the time of birth, the newborn is confronted with the mother’s vaginal microbes and environmental organisms. The infant’s skin surface is initially colonized followed by the oropharynx, gastrointestinal tract, and mucosal surfaces.
Microbes that have the ability to cause serious diseases are normally found in and on the human body. Not the recovery of specific organism, but the recovery of the organism in a nor-mally sterile site is the hallmark of pathogenesis of microbial infections. For example, Escherichia coli is a normal resident confined to the gastrointestinal tract. If it is demonstrated in the stool, it may be considered normal, but if found in the abdominal cavity or the patient’s blood stream, this would be considered abnormal. Similarly, there are certain organisms that are never present as part of the normal microbial flora in humans; hence their recovery in humans is always associ-ated with clinically significant diseases (e.g., Bacillus anthracis, Brucella spp.,Francisella tularensis, and Histoplasma capsulatum,etc.).
Viruses and parasites are not considered as members of the normal microbial flora because they are not found as commen-sals in the host.