Normal Flora: Introduction, Types, Beneficial and Harmful Effects

Normal flora

Introduction of Normal Flora

Flora meaning from Cambridge  English Dictionary:-In the normal, healthy person the gut flora is a balanced community of different organisms. Normal flora refers to the population of microorganisms that inhabit the skin and mucous membranes of a normal body. A healthy fetus in-utero is free from microorganisms. During birth, the infant is exposed to vaginal flora. Within few hours of birth, oral and nasopharyngeal flora develops, and in a day or two resident flora of the lower intestine appears.

Types of normal flora

The normal microbial flora is more or less constant that can be divided into two groups-

  1. Resident flora: It consists of organisms that are regularly present in a particular area and when disturbed it re-establishes itself. e.g. Escherichia coli is a normal inhabitant of the intestine.
  2. Transient flora: It consists of both non-pathogenic and potentially pathogenic bacteria that inhabit the body surface or mucous membrane for a limited period. They can be eliminated from the body surface by mechanical means. Pathogens such as the pneumococcus and the meningococcus may be found in the nasopharynx of humans from time to time.

Normal  flora of body sites

  1.  Skin: Micrococcus species, Corynebacterium species, Staphylococcus epidermis
  2. Mouth: bacteria of Enterobacteriaceae family, Lactobacillus species, Staphylococcus epidermis, Staphylococcus aureus, Neisseria species, Corynebacterium species, Haemophilus species
  3. Nose and nasopharynx: Neisseria species, Corynebacterium species, Staphylococcus aureus, Staphylococcus epidermis, Haemophilus species, Streptococcus pneumoniae
  4. Conjunctiva: Neisseria species, Corynebacterium species, Staphylococcus epidermis, Haemophilus influenzae, Viridans streptococci
  5. Large intestine and lower ileum: Staphylococcus species, Lactobacillus species, Clostridium species, Escherichia coli, Proteus species, Enterobacter aerogenes, Klebsiella pneumoniae, Pseudomonas aeruginosa, Mycoplsma species, Candida albicans, Enterococcus faecalis.
  6. External ear: Corynebacterium species, Staphylococcus epidermis, Staphylococcus aureus.
  7. Urethra: Candida species, Mycoplasma species, Mycobacterium smegmatis, Acinetobacter species
  8. Vaginal (between puberty and menopause-Acid pH): Lactobacillus species, Streptococcus species, Clostridium species, Corynebacterium species, Gardnerella vaginalis.
  9. Before puberty and after menopause(alkaline pH): Staphylococcus epidermis, Micrococcus species, Enterobacter species, Corynebacterium species, Viridans streptococci

Beneficial Role of Normal Flora

  1. They prevent or suppress the colonization/invasion of the body by pathogens.
  2. The bacterial flora of the intestinal tract synthesize vitamins, especially K and several B vitamins.
  3. Antibodies produced in response to commensals cross-react with pathogens having related or shared antigens and thus raise the overall immune status of the host against pathogens.
  4. Colicins, produced by some organisms of normal flora, have a harmful effect on pathogens. The endotoxins liberated by them may help the defense mechanism of the body by triggering the alternative complement pathway.

Harmful Effects of Normal Flora

  1. They become pathogenic when the host resistance is lowered.
  2. They may act as pathogens in tissue outside their normal habitat e.g. normal flora of the intestine may cause urinary tract infection.
  3. Streptococcus mutants may cause dental caries.
  4. Penicillinases producing organisms can aggravate infection by interfering with therapy.
  5. The use of broad-spectrum antibiotics affects the normal flora by inhibiting sensitive bacteria and thereby allowing overgrowth of resistant bacteria.
  6. Normal flora may cause confusion in diagnosis due to their ubiquitous presence in the body and their resemblance to some of the pathogens.

Some video clips of micro-organisms related to Normal Flora are as follow-

Neisseria meningitidis in blood culture as shown below-

Micrococcus species as shown below-

Corynebacterium species as shown below-

Staphylococcus and Enterococcus growth on blood agar as shown below-


Haemophilus influenzae growth on chocolate agar as shown below-



Streptococcus pneumoniae growth on 5% sheep blood agar and showing organism sensitivity towards the 5µg optochin as shown below-


Viridans streptococci growth on blood agar as shown below-


Lactobacillus growth on chocolate agar and its gram stain pictures as shown below-

Growth of Clostridium on blood agar and its Gram stain pictures as shown below-

Proteus growth on blood agar showing swarming growth as shown below-

Klebsiella pneumoniae mucoid colony characteristics after 2 days of incubation as shown below-

Pseudomonas aeruginosa colony morphology on nutrient agar showing two pigments ( pyoverdin and pyocyanin) as shown below-

Various Candida species growth on chromagar as shown below-


Further Readings

  1. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  2. Clinical Microbiology Procedure Handbook Vol. I & II, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  3. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr and Sommers H.M.
  4. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  5. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  6.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
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