Medical Microbiology

Normal Microbiota Definition, Types, Advantages and Disadvantages.

THIS BLOG INCLUDES: hide 1 Normal Microbiota definition 2 Types of Normal Microbiota 3 Normal Microbiota of Human Body 4 Advantages/Benefits of...

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This article writter by SouravBio on June 07, 2020

Writer and Founder of I am from India and my main purpose is to provide you a strong understanding of Microbiology.

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Normal Flora of Hman Body

We are all familiar with the term Normal Microbiota or Normal Microbial Flora or Human microbiome, but we don’t know the real benefits of normal microflora, so in this article, I will discuss the benefits of normal microbial flora in the human body, and how they benefit us.

First of all, we have to know What is Normal Microbiota? so I will discuss every detail about microflora step by step if you face any problem then feel free and comment in the below section.

All the microorganisms are not harmful for example, our body surface contains different varieties of microorganisms in a large number, which are provides benefits to our body.

The normal microbiota of a human body is not static. They are started to colonize in our body after birth, and they constantly change with the human ages.

When a new-born comes in contact with the environment, microorganisms instantly started to colonize in the body surface. After that, the number of microorganisms is started to increase constantly with the human age as you can see in this below table.

Normal Microbiota definition

Normal Microbiota defines as those microorganisms which are present in the human body in a huge number.

Our environment contains a huge number of microorganisms, whenever we exposed to the environment our body surface gets in contact with those microorganisms, and they are started to colonize in our body surface. Except for surface microflora, microorganisms also present in our vital organs, including the liver, gastrointestinal tract, mouth, urinary tract, etc. In the case of a healthy person, all the initial tissues (blood, brain, muscle, etc) are normally free of microorganisms.

In one sentence, all the microorganisms are present in our anatomical sites, are defined as Normal Microbiota.

The Normal Microbiota of microorganisms is consisting of eukaryotic fungi, protists, bacteria, etc. Among them, bacteria are present in large numbers.

Types of Normal Microbiota

There is a present different type of Normal Microbiota in the human body, including bacteria, fungi, archaea, viruses.


The initial studies show that an individual contains thousands of bacteria on their body surface and vital organs. The mouth and the gut portion of the human body contains a large number of bacteria, which is greater than the bacterial population of Skin and vaginal sites. There are present different species of bacteria that are found in the human body, which are vary based on the type of organs.


Archaea mainly found in the gut portion of the human body. The species of archaea is much more limited in our body.


They are mainly found at the gut portion and skin of the human body.


Viruses, including bacterial viruses (bacteriophages), are colonize in our various body parts, such as skin, gut, lung, oral cavity, etc.

Normal Microbiota of Human Body

Microorganisms are present in different parts of our body such as skin, eye, nose, mouth, gastrointestinal tract, etc. We discussed the normal microbiota of each body part in below.

Normal Microbiota of Human Body
Normal Flora of Human Body

Normal Microbiota of Skin

An adult human body has approximately 2 m2 of skin, and it contains about 10^12 numbers of bacteria.


There are present different types of bacteria on our skin surface. The diversity of bacteria is vary based on the different locations of our skin. Our skin is divided into three parts: The dry portion, the moist portion, and sebaceous (containing sebum).

The dry portions of our skin include forearm, buttocks, hands, etc. This portion contains the greatest diversity of gram-negative and gram-positive bacteria, such as Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria.

The moist areas of our skin include umbilicus, underarm, inguinal and gluteal creases, and inside the elbow, etc. These portions of the skin exhibit less diversity of microorganisms. It contains mostly some Firmicutes and Actinobacteria such as Staphylococcus and Corynebacterium spp.

The oily sebaceous sites contain the lowest diversity of bacteria including most members of Actinobacteria (Propionibacterium spp.). The oily portions of our skin include the forehead, behind the ear, and the back.

Some occasionally pathogenic bacteria are present in our skin surface such as;

  • Staphylococcus epidermidis
  • Staphylococcus aureus
  • Staphylococcus warneri
  • Streptococcus pyogenes
  • Streptococcus mitis
  • Cutibacterium acnes
  • Corynebacterium spp.
  • Acinetobacter johnsonii
  • Pseudomonas aeruginosa


Except bacteria, our skin also contains more than 14 different genera of fungi, including;
dermatophytes (skin living fungi) such as Microsporum gypseum, and Trichophyton rubrum and nondermatophyte fungi (opportunistic fungi that can live in the skin) such as Rhizopus stolonifer, Trichosporon cutaneum, Fusarium, Scopulariopsis brevicaulis, Curvularia, Alternaria alternata, Paecilomyces, Aspergillus flavus and Penicillium species.

Normal Microbiota of Nose, and Pharynx.

The nasopharynx is a part of the pharynx lying above the level of the soft palate. The Normal Microbiota of Pharynx is including some potential pathogenic bacteria such as Streptococcus pneumonia, Neisseria meningitides, and Haemophilus influenza. Also, a large number of non-pathogenic Gram-positive bacteria are commonly found in both the nose and nasopharynx.

Normal Microbiota of Mouth

The mouth has an optimum environment (water, nutrients, neutral pH, and moderate temperature) which will help in the growth of Microorganisms. The mouth portion of the human body contains mostly the genera of some Streptococcus, Neisseria, Actinomyces, Veillonella, and Lactobacillus, as well as some yeasts.

Normal Microbiota of Throat

The microbial flora of throat is including non-hemolytic and alpha-hemolytic streptococci, some Neisseria species, staphylococcidiphtheria and Hemophilus organisms, pneumococci, yeasts, and Gram-negative rods.

Normal Microbiota of Respiratory Tract

Our lower and upper portion of the respiratory tract is free of microorganisms because there are three important reasons such as;

  1. A continuous stream of mucus entraps the microorganisms, and the ciliated epithelial cells continually move the entrapped microorganisms out of the respiratory tract.
  2. Alveolar macrophages destroy the microorganisms.
  3. A bactericidal effect is exerted by the enzyme lysozyme, present in the nasal mucus.
    The enzyme lysozyme (nasal mucus) shows a bactericidal effect that kills all the bacteria.

Normal flora of Eye

A small number of bacteria are present in eye including;

  • Chlamydia trachomatis
  • Chlamydophila pneumonia
  • Haemophilus aegyptius
  • Haemophilus influenza
  • Moraxella app
  • Neisseria app
  • Staphylococcus aureus
  • Staphylococcus epidermidis
  • Streptococcus viridians.

Normal Microbiota of Gastrointestinal Tract

Due to the presence of very low acidic pH (2 or 3), the stomach contains few bacteria including Streptococcus, Staphylococcus, Lactobacillus, Peptostreptococcus app.

The small intestine also contains several bacteria such as; Enterococcus faecalis, lactobacilli, diphtheroid, and the yeast Candida albicans are occasionally found.

The large intestine contains a huge population of microorganisms. More than 400 species of bacteria were isolated from human feces. Most of them are anaerobic, Gram-negative bacteria, and Gram-positive rods. Al those bacteria belong to the genera of Bacteroides, Clostridium, Faecalibacterium, Eubacterium, Ruminococcus, Peptococcus, Peptostreptococcus, and Bifidobacterium.

There also present several fungi such as Candida, Saccharomyces, Aspergillus, Penicillium, Rhodotorula, Trametes, Pleospora, Sclerotinia, Bullera, and Galactomyces.

Normal Flora of Urinary tract

Both the male and female urinary tract contains a huge number of microorganisms such as;

1.Microbial flora of Male Urinary tract or Distal Urethra

Male Urinary tract contains these following microflora;

  • Coagulase-negative staphylococci
  • Diphtheroid
  • Streptococcus spp.
  • Mycobacterium spp.
  • Bacteroides spp.
  • Fusobacterium spp.
  • Peptostreptococcus spp.

2. Microbial flora of Female Urinary tract Vigna

Female Urinary tract contains these following microflora;

  • Lactobacillus spp.
  • Peptostreptococcus spp.
  • Diphtheroid
  • Candida spp.
  • Gardnerella spp.
  • Bacteroides spp.
  • Streptococcus spp.
  • Clostridium spp.

Advantages/Benefits of Normal Microbiota to the Human body

The Normal Microbiota of the human body provides serval benefits, which are discussed below;

1.Prevent Pathogens

Most importantly Normal Microbiota prevents the colonization of Pathogens by competing with them for essential nutrients and attachment sites.

2. Vitamin Synthesis

Some normal microflora synthesis vitamins for their own need. Sometimes our body absorbs those vitamins as a nutrient.

Example: Some enteric bacteria of the human body produce Vitamin K and Vitamin B12. Lactic acid bacteria also produce certain B-vitamins.

3. Killing bacteria

Some normal flora antagonizes other bacteria by producing some substances which inhibit or kill nonindigenous species.

Example: few intestinal bacteria produce some substances which have highly specific bacteriocins, that helps to kill other bacteria.

4. Development of certain tissues

Normal flora also helps in the development of certain tissues.

Example: Caecum and certain lymphatic tissues (Peyer’s patches) in the gastrointestinal tract.

5. Production of Antibody

The normal flora of the human body also helps to stimulate the synthesis of normal antibodies.

Example: When The normal flora acts like an antigen in our body, they induce an immunological response, in particular, an antibody-mediated immune (AMI) response, which induces the production of natural antibody.

Disadvantages of Normal Microbiota

There are present several harmful effects normal flora on the human body;

1. Bacterial synergism

This means that normal flora is helping another potential pathogen to grow or survive. There are present few normal florae that supply vitamins and nutrients to a pathogen for growing and survive, which is known as cross-feeding between microbes.

For Example, During the treatment of “staph-protected infections”, a penicillin-resistant staphylococcus which is a component of the normal flora shares its drug resistance properties with pathogens that are otherwise susceptible to the drug.

2. Competition for nutrients

The normal flora absorbs vitamins and nutrients from the gastrointestinal tract of animals for their own needs.

3.Toxin production

Some bacteria of normal flora produce toxic compounds in the host body, which results in some harmful effects.

4. Disease

Sometimes normal flora can cause disease in the host body. If somehow the normal defense system or immune system of a host body is unable to restrict or tolerate the normal flora then they can cause diseases in the host body.


The Normal Microbiota has a limitation in the human body. They never harm the host body, but in some cases, if the host body’s immune system becomes weak than that normal flora can cause disease.

MCQ related to Normal Microbiota

Which statement regarding normal microbiota of the human respiratory system is FALSE?

a. The upper respiratory tract is mostly sterile.(Ans)
b. The ciliary escalator in the bronchial tubes keeps the lower respiratory tract nearly sterile.
c. Potentially pathogenic microorganisms are present in nonsterile portions of the respiratory tract but usually do not cause disease.
d. The trachea normally may contain a few bacteria.

Which of the following is NOT caused by infection of the upper respiratory tract?

a. diphtheria
b. scarlet fever
c. streptococcal pharyngitis
d. whooping cough(Ans)

Which of the following situations is NOT a way in which a baby acquires normal microbiota?

a. microbes cross the placenta during pregnancy.(Ans)
b. microorganisms grow in the respiratory tract after the baby’s first breath.
c. microbes enter the nose and mouth when the baby is in the birth canal.
d. The baby acquires the residential microbiota in the colon after the first meal.

Which of the following situations might cause normal microbiota to become opportunistic pathogens?

treatment of a cancer patient with radiation

Which statement regarding normal microbiota of the urinary and reproductive systems is true?

a. Normal urine has a high level of normal, nonpathogenic microbes.
b. Pregnancy and menopause are associated with higher rates of urinary tract infections. (Ans)
c. Candida albicans is the predominant bacterium in the vagina.
d. The male urethra is usually highly contaminated with microbes.

Women and men typically have the same normal microbiota in their reproductive systems.

a. True
b. False (Ans)

Normal microbiota provide protection from infection in each of the following ways EXCEPT

A) they produce antibacterial chemicals.
B) they compete with pathogens for nutrients.
C) they make the chemical environment unsuitable for nonresident bacteria.
D) they produce lysozyme.(Ans)
E) they change the pH of the environment.

Which of the following would NOT be a likely member of the normal microbiota of the skin surface?

a. obligate anaerobe (Ans)
b. lipophile
c. acidophile
d. halophile

Microorganisms that are members of the normal microbiota are also known to cause disease.

a. True (Ans)
b. False

Normal microbiota may cause disease if they are introduced into an unusual site in the body.

a. True (Ans)
b. False

Which statement about protective mechanisms employed by normal microbiota is false?

a. Normal microbiota consume nutrients.
b. Normal microbiota stimulate the second line of defense.
c. Normal microbiota spreading to locations where they are not normally found to prevent colonization from pathogens. (Ans)
d. Normal microbiota alter the pH of their environment.

How do normal microbiota and transient microbiota differ

Normal microbiota is always present on or in the body. Transient microbiota is not always present and most of the time is pathogenic.

Under normal circumstances, the microbiota of the skin is kept in check by

A. a pH above 8.
B. a slightly acidic pH. (Ans)
C. phagocytic activity.
D. excessive moisture.

All of the following are normal microbiota of the skin except

A) Streptococcus.(Ans)
B) Micrococcus.
C) Staphylococcus.

Which of the following can result when antibiotic therapy disrupts the normal microbiota?

A) anaphylactic shock
B) black hairy tongue
C) pseudomembranous colitis
D) both pseudomembranous colitis and thrush (Ans)

Why is Mannitol salt agar used as a selective medium for normal skin microbiota?

Mannitol salt agar is used as a selective medium for normal skin microbiota because most bacteria on the skin are salt-tolerated and will grow on an MSA plate.

Normal microbiota are typically found in and on all the following body locations except the

Blood (ans)

Normal microbiota may cause disease if conditions change in the body.

a. true (ans)
b. false

Most of the normal microbiota of the digestive system are found in the

a. large intestine (Ans)
b. mouth
c. small intestine
d. stomach
e. stomach and small intestine

Normal microbiota of the adult vagina consist primarily of

A) Lactobacillus. (Ans)
B) Streptococcus.
C) Mycobacterium.
D) Neisseria.
E) Candida.

Which statement correctly describes the normal microbiota of the skin?

a. The skin’s normal microbiota are capable of growth at elevated salt concentrations. (Ans)
b. Most microorganisms that grow on the skin are sensitive to drying.
c. Vigorous washing will eliminate all skin bacteria.
d. On superficial skin surfaces, certain anaerobic bacteria produce fatty acids from oil gland secretions.

Normal microbiota play a number of essential roles, including:

Preventing disease by competing with disease causing microbes, helping degrade foods that would otherwise be indigestible, promoting immune system development and causing disease.

Is there Normal Microbiota present in the Lower respiratory system?

Typically there are no microorganisms present.

which of these would NOT describe characteristics of the normal microbiota of the skin?

a. gram-negative (Ans)
b. gram-positive

which is not a characteristic of the normal microbiota?

a. are often called the normal flora
b. live within our bodies
c. live on our bodies
d. may benefit the human host
e. regularly associated with disease symptoms (Ans)

Normal skin microbiota are able to grow on the skin because they can thrive in the presence of

A) sebum.
B) salt.
C) keratin.
D) sebum and salt. (Ans)
E) sebum, salt, and keratin.

Normal microbiota provide protection from infection in each of the following ways EXCEPT

A) they produce antibacterial chemicals.
B) they compete with pathogens for nutrients.
C) they make the chemical environment unsuitable for nonresident bacteria.
D) they produce lysozyme. (Ans)
E) they change the pH of the environment

Normal microbiota helps to __ colonization of pathogenic organisms.

-prevent (Ans)

Both human pathogens and normal microbiota are typically classified as:

a. mesophiles (Ans)
b. psychrophiles
c. thermophiles
d. hyperthermophiles

Which of the following is true about the normal microbiota of the nervous system?

A) Only transient microbiota are present.
B) There are no normal microbiota. (Ans)
C) Normal microbiota are present in the central nervous system only.
D) Normal microbiota are present in the peripheral nervous system only.
E) None of the above.

Normal skin microbiota are able to grow on the skin because of their resistance to:

-sebum, salt, and keratin.
-sebum and salt. (Ans)

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