Normal flora of human host: Types, Examples and Roles
- Microorganisms associated with healthy tissue of host semi-permanently without causing disease are known as Normal flora or Microbial flora or Human microbiota.
- All human tissue which are directly or indirectly exposed to external surrounding have normal flora. Such tissue in Human includes skin, GI tract, urinary tract, respiratory tract, eyes etc.
- Internal organs such as blood, kidney, heart, lungs, brain etc are sterile in normal healthy individuals. Therefore, if microorganism is found in these internal organs it is referred as diseased condition.
- Healthy fetus in uterus is sterile. Child first gets its normal flora during birth by contact with vaginal flora of mother. After birth additional microbial flora are established through the sources of food, water and air.
Examples of normal flora:
Skin flora: Staphylococcus epidermidis, S. aureus, Propionibacterium acne,
GI tract: Lactobacillus, Helicobacter pylori, Enterococcus, Bacteroids, Bifidobacterium, E. coli, Streptococcus faecalis
Respiratory tract: Streptococcus, Corynebacterium, Haemophilus, Micrococcus,
Normal flora of eye:
- Continuous flow of tear over conjunctiva removes many microorganism. Furthermore tear contains lysozyme which is antimicrobial agent. Therefore very few microorganisms are found on conjunctiva.
- Some predominant microflora are; Staphylococcus epidermidis, Corynebacterium spp, Streptococci, Neisseria spp, Moaxella spp, Haemophilus parainfluenzae
Types of normal flora:
- Commensal microflora:
- Mutualistic microflora:
- Opportunistic microflora:
Commensal microflora:
- These microorganisms gets benefits from human host and give neither benefits nor harmful effects to host.
- Majority of normal flora are commensal types
Mutualistic microflora:
- These microorganisms gets benefits from host and at the same time give benefits to the host.
Opportunistic microflora:
- These microorganisms, under normal condition live as commensal on host body but they causes diseases if opportunity is available.
- For example; Normal flora of GI tract ie. E.coli causes urinary tract infection (UTI), if the site of habitat is changed.
Role of normal flora:
Beneficial role of normal flora;
- Prevent attachment and penetration of pathogenic microorganisms: Normal flora prevent attachment and penetration of pathogenic microorganism through skin and other tissue as they occupy the area. Some normal flora produces mucin and make the surface slippery so that pathogenic microorganism cannot attach to cause disease.
- Compete with pathogenic microorganisms: Microflora compete with pathogenic bacteria for habitat and nutrition.
- Produces antibiotics: Some normal flora produces antimicrobial chemicals (antibiotics) that kills pathogenic microorganism. For eg, E. coli produce Cloicine in intestine of human that kills many pathogenic bacteria.
- Immunity: Antibodies produces against normal flora can neutralize pathogenic microorganism and prevent infection.
- Produces enzymes and vitamins: Some intestinal normal flora produces useful substance for host such as vitamins and digestive enzymes. Eg. E. coli produces Vitamin B12 and Vitamin K.
- Helps in metabolism: Intestinal normal flora produces enzymes such as cellulose, galactosidase, glucosidase etc and helps in digestion of food.
- Oxidation and hydrolysis of steroids: Intestinal microflora also helps in metabolism of steroids. For eg. Bacteria in intestine carry out oxidation and hydrolysis of steroid ring of bile salt.
Harmful effects of normal flora:
- opportunistic infection: Normal flora may causes opportunistic infection when immunity of host become weak or if normal flora of one tissue migrates to other habitat. For eg. If E. coli of GI tract migrates to Urinary tract it causes UTI.
why Staphylococcus aureus is an opportunistic pathogen?
Answer:
Staphylococcus aureus is an opportunistic pathogens of skin. In normal condition S. aureus lives as commensal in skin but it causes opportunistic wound infection when skin is damaged by burn, wound, biting, scratching etc. form the damaged skin, bacteria enters into deeper tissue and multiplies to give pus filled lesions.
Staphylococcus aureus is also as opportunistic pathogen of upper respiratory tract.in normal condition, S aureus is commensal in nasal chamber but it causes secondary bacterial pneumonia in immunocompromised host. Patient infected with influenza or measles virus infection are at very high risk of S. aureus infection. Virus infection damages the mucus lining of respiratory tract so that deeper soft tissue is exposed to commensal flora ie Streptococcus aureus which penetrates and causes secondary bacterial infection.