They are mostly found in the skin, oropharynx, colon, and vagina.
Viruses and parasites are not considered part of the normal flora but they may be present in asymptomatic patient.
The following areas of the body are free from normal flora and are normally sterile:
- Central Nervous system
- Lower Bronchi
- Urinary Bladder
1. They interfere with the colonization of pathogenic bacteria and also secretes antimicrobial agents that inhibit destroy the pathogenic bacteria. Thus they contribute to the immune system of the host(man).
2. The normal flora helps the development of the immune system in case of neonates.
3. The normal flora in the intestine synthesize vitamin B and vitamin K.
Harmful effects of normal flora:
1. They are opportunistic pathogens. This means that they do not cause disease in a normal immuno-competent person but if for some reason the person is immuno-compromised (eg AIDS patient, Malignancy) or immuno-suppressed (long term steroid therapy in autoimmune disease, organ transplantation), then they can become pathogenic and cause diseases.
2. If the normal flora of one part of the body migrate to another part of the body which is not its normal site of residence then it can cause disease. eg, E. Coli bacteria can cause urinary tract infection if it migrates from the gut to the urinary bladder.
Read about the normal flora of skin.
In the making of this article the following book was used:
1. Review of Medical Microbiology and Immunology, Tenth Edition (LANGE Basic Science)