Normal flora in the Human body: Its Origin, Classification, Significance and Occurrence

The phrase ‘nature is around and within us’ is commonly used in many cultures. This statement couldn’t be truer when we analyze the ‘normal flora,’ also known as the ‘natural microbiota,’ that inhabits the human body. The normal flora refers to microbes, mostly bacteria, that inhabit the skin and mucous membranes. The role of these tiny organisms encompasses various functions, including defense and immune system support, as well as digestion and nutrient production.¹ Although most of these organisms are mostly harmless or even beneficial, some of them may be opportunistic pathogens leading to various infections.

Origin of normal flora

A healthy human fetus is free of bacteria. The fetus acquires microorganisms while passing through the birth canal. These microbes are continuously joined by other microbes found in the immediate surroundings. On finding a suitable environment, they quickly multiply and form a stable natural flora. Each part of the human body plays host to different microbial populations. The number and composition of this flora may fluctuate depending upon diet, general health, hormonal activity, age, and many other factors.¹

Classification of normal flora

Normal flora can be classified into two general types based on their location, function, and duration of presence in the body. They are as follows:

Resident normal flora

These microflora are regularly found in a given area at a given age in fixed proportions. It promptly reestablishes its population when disturbed. It is acquired rapidly during as well as after birth.

Transient normal flora

It includes microbes that inhabit the body surface or mucous membrane for a limited period. These microbes can be removed from the body surface by various mechanical means.2 These include saprophytes, commensals, as well as facultative pathogens.

Occurrence of Normal flora in different areas of the human body

Normal flora is found in almost all parts of the human body, specifically in areas such as the skin, gastrointestinal tract, and eyes. However, blood, kidney, cerebrospinal fluid, and brain are sterile, i.e., free from any type of normal flora. A characteristic group of microbes resides in different parts of the human body. It has been estimated that the human body consists of microbial cells ten times the number of eukaryotic cells.¹Some of the human body parts and their characteristic microbial inhabitants are as follows:

Fig: Normal flora at different body sites. (Source: Basic Medical Key. Human non-sterile sites and normal flora in health. In: Medical Microbiology)

Skin

The most common bacteria making up the skin flora are Staphylococcus, Micrococcus, and aerobic Corynebacterium.

Conjunctiva

Lysozyme secreted along with the tears maintains a sparse microbial population in the conjunctiva. However, some bacteria, such as Corynebacterium, Neisseria, Moraxella, S. epidermidis, Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae, may still be found occasionally.

Mouth

The predominant bacterial species that make up the microbiota of the mouth are Streptococcus, Neisseria, Veillonella, Actinomyces, and Lactobacillus.

Upper respiratory tract

S. epidermidis and S. aureus are most consistently found in the nose. S. pneumoniae and Streptococci are predominant in the nasopharynx.

Intestinal tract

The normal flora living in the intestines, also known as the ‘gut microbiota’, has been of significant interest to medical researchers, attributing to their role in digestion and vitamin synthesis. Some of the most prominent members among them are Bacteroides (B. fragilis, B. melaninogenicus, and B. oralis), Fusobacterium, Bifidobacterium, Eubacterium, Lactobacillus, Escherichia, Proteus, Klebsiella, Enterobacter, and Peptostreptococci.

Genitourinary tract

Lactobacillus is the most predominant member of the normal flora found in the vagina of young females. After puberty, Corynebacteria, Peptostreptococci, Staphylococci, and Streptococci are found to be predominant.¹

Significance of Normal Flora

Normal flora compete with the pathogens for nutrition and space, suppressing their entry into the body.
Evidence supports that vitamin K and several group B vitamins are synthesized by gut microbiota.
Normal flora can activate the immune system of the host against the pathogens by producing of corresponding antibody.
Some microbiota may release endotoxins, which help the defense mechanism of the body.
Gut microbiota plays a significant role in digestion and absorption.
Normal flora may also take part inthe breakdown of toxins and drugs.

Dysbiosis

Dysbiosis refers to the phenomenon in which the normal composition of the body’s microbiota is disturbed or altered. It can involve loss of beneficial microbes, overgrowth of potentially harmful bacteria, or reduction in microbial diversity.2 It occurs due to several factors such as antibiotic use, poor diet, stress, infections, environmental exposures, etc. This imbalance can lead to several health problems. Some of them are as follows;

Inflammatory Bowel Disease (IBD)
Irritable Bowel Syndrome(IBS)
Bacterial vaginosis
Urinary Tract Infections(UTIs)
Food intolerances
Acne
Dental caries

Role of normal flora in modern medicine

Human microbiota has been a significant topic of research in the field of modern medicine. The role of normal flora in health, disease prevention, and treatment is significantly recognized. Probiotics and prebiotics are used to restore microbial balance. The composition of an individual’s microbiota is used as a biomarker for the development of personalized treatment plans and disease preventive measures. Advancements in research can enable the production of engineered probiotics and microbiome editing techniques, which may be used in the treatment of chronic and immune-related diseases.

REFERENCE

Pelczar MJ, Chan ECS, Krieg NR.Microbiology: Concepts and Applications. New York, NY: McGraw-Hill; 1993.
Tortora GJ, Funke BR, Case CL. Microbiology: An Introduction. 12th ed. Boston, MA: Pearson Benjamin Cummings; 2016.