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Influence of Intestinal Microbiota in Human Health and Behavior

Updated on August 5, 2016
Escherichia coli: a long-term resident in our gut
Escherichia coli: a long-term resident in our gut | Source


Several decades ago, living things are classified into two kingdoms, namely animal and plants. In early stages, a microorganism is classified under natural vegetation. That is why it is known as flora. In recent decades, the microorganism is classified in its own kingdom, so the term flora is omitted. The term microflora (Origin: microbial flora, referring to microbes that inhabit intestines) is still being used, but gradually being replaced by the term microbiota.


Microbiota refers to microbes that live in a particular place. Many people confuse whether the term microbes has a similar meaning to microorganisms. The answer is Yes, correct. It is similar and can be used interchangeably. However, it is typically used to refer to pathogens (agents that cause disease).

Recently, a term microbiome is used by scientists to refer to a variety of microbes that inhabits the human body.

Many scientific articles have distinguished between microbiome and microbiota by explaining that microbiome refers to the collective genome of a microorganism occupying an environment whereas microbiota is described as the microorganism itself. However, based on the original definition of the term, both are synonymous. So far, various scientific articles using the term microbiota, which generally refers to bacteria. The use of the term is based on bacteria as the largest occupant than archaea and fungi.

Yersinia enterocolitica
Yersinia enterocolitica | Source


To avoid confusion, the term microbiota will be used in subsequent paragraphs.

The main microbiota that inhabits the human body including the colon is bacteria, archaea and fungi (the most common type of fungi studied is Candida species). Archaea belong to the prokaryote, which has no cell nucleus or any organelle. Previously, it was classified as bacteria and called archaebacteria (Archaebacteria kingdom) but the classification was dropped.

Microbiota occupies the body parts exposed to the environment, including the skin, saliva, mouth, eye (on the conjunctiva) and vagina, but the vast majority of it inhabits the intestine where ample nutrients are consistently available.

Key Roles

Today, scientists find a variety of key roles of microbiota to human health. Practically almost every scientific study conducted in an effort to try to show a link between microbiota and specific properties or diseases has resulted in success.

So far, studies involving microbiota (or the downside) have demonstrated to nutrient imbalances, obesity, heart disease, diabetes, colitis, celiac disease, multiple sclerosis, eczema, most types of cancers, and autism. In addition to helping digestion and detoxification, intestinal microbiota (especially bacteria as studies mostly focus on it) is also found to influence the immune system.

The Invisible Universe Of The Human Microbiome

Robust Immune System

In infancy and toddlerhood, bacteria play a role in building a comprehensive immune system, during which the immune system gets familiar with foreign antigens in the body (better known as ‘non-self’) and subsequently being tolerance to the same antigens in the future. When homeostasis forms, non-pathogenic microbes and harmless microbes will not trigger inflammation. Indeed, certain types of inflammation are associated with autoimmune diseases and allergies, both of which involve the immune system.

Vaginal Birth

A study shows hygienic rats have a subefficient immune system. The rats in the study were found suffering from autoimmune diseases and shows bad traits. The result of the study infers that the bowel microbiota in infants is capable of creating a lasting impact on one health. Scientists have also been comparing babies born by caesarean surgery (C-section) with babies born vaginally and they found that microbiota occupying C-section babies is microbiota of his mother's skin while babies born vaginally is occupied by their mother’s vaginal and intestinal microbiota. As a result, C-section babies prone to allergies and obesity compared with vaginal birth babies.

Nutrition and Obesity

Studies have also shown the role of the intestinal microbiota in nutrition and obesity. Intestinal bacteria are responsible digesting complex molecules of food such as meat and vegetables. By metabolizing food eaten by human, bacteria obtain energy for their use. Studies on rats showed that some bacteria are linked to obesity and others with normal weight. To the researchers’ amazement, when obese rats fed on intestinal microbiota of normal rats, the former experienced weight loss. On the other hand, when normal rats fed on intestinal microbiota of obese rats, the former gained weight.

Signalling molecules

Another study involving human twins with different weight, raised in a similar way and have identical genome shows similar links between weight and microbiota. The study also shows a direct link between diet and high levels of certain types of gut microbial community. For instance, vegetarians were found to have intestinal microbiota with the ability to digest plant cellulose, molecules usually not easily digested by humans.

When the bacteria carry out metabolism of complex molecules, signaling molecules are produced and signals sent to the brain via vagus nerve – act that affects behavior. This discovery led some scientists to believe that intestinal microbiota may have a role of our craving for certain types of food apart from its role in influencing the selection of food.

Gastrointestinal Diseases

The intestinal microbiota is also found to be involved in various diseases of gastrointestinal diseases especially those based on inflammatory symptoms including Crohn's disease, ulcerative colitis, and irritable bowel syndrome.

Any of those diseases that weaken the body was found to have a strong association with the changing diversity of the gut bacteria (microbiota major group)

Even though the diversity of the microbiota is usually in a state of balance, however, taking antibiotics (drugs that kill bacteria) as well as prolonged diarrhea said able to change the balance of the diversity, which stems from the residential of new types of bacteria.

The population of Clostridium difficile which is usually low in normal intestinal condition, for example, said to be soaring caused by antibiotics use, which eradicates all existing bacteria that live to compete with them. Infection of C. difficile causes diarrhea and flu-like symptoms, which may result in death if not treated well.

Fermented and Cultured Foods

C. difficile found to be effectively controlled by transferring microbiota of healthy donor to the intestine of infested individual. When a new population of bacteria reproduces perfectly in the intestine, the patient will recover.

From an alternative medicine perspective, lifestyle changes including avoiding ultra-processed foods, manage stress diligently, avoid alcohols (as it kills off friendly microbiota), taking modern drugs especially antibiotics when really needed, and taking plentiful vegetables and fruits may help keep intestinal microbiota in a healthy balance (proper ratio between friendly and bad bacterial colonies).

Taking probiotics in the form of fermented foods and cultured milk or dairy products might also be very helpful.

Scanning electron micrograph of en:Clostridium difficile bacteria from a stool sample
Scanning electron micrograph of en:Clostridium difficile bacteria from a stool sample | Source

Effect on Behavior

Recent studies also found that intestinal microbiota affects the behavior of its host. There are numerous nerve endings located in the intestines that receive and subsequently transmit signals in the form of metabolites and small molecules secreted by bacteria to the brain directly via the vagus nerve. The metabolites and small molecules secreted by bacteria are believed to affect many things including the taste of food and mood.

In a study in which scientists replace the microbiota of mice of risk takers with the microbiota of normal mice has produced a cowardly behavior being afraid of taking risks. A hypothesis based on other studies also shows that our crave for certain foods and flavors can also be determined by the diversity of certain types of intestinal bacteria.

Preference for Specific Foods

It is believed that certain types of bacteria use only specific foods as a power source. In this context, the moodiness and the autistic spectrum disorder (ASD) are some of the symptoms involving aspects of conduct are also associated with the microbiota

Scientists already know there is a strong link between the gut (due the presence of microbiota) and behavior. Studies also show that autistics experience irritable bowel disease and some kind of intestinal disorders.


The study focused on the hypothesis based on these studies that intestinal microbiota dysbiosis believed to have a link with autism. The scientists of the study induced the symptoms of ASD in rats using a method in which mother rats were infected with molecules resemble viruses which caused rat pups to develop symptoms of ASD very easily.

The procedure directly shows that rat pups with symptoms of ASD experience low integrity of gastrointestinal track compared to the rat pups from mother rats who experienced no symptoms of ASD. In addition, increased of some of the metabolites was believed to be biomarkers to autism in humans.

Lastly, when the rat pups with ASD were fed probiotics, symptoms found to be decreased along with a decrease in metabolite levels. A conclusion that can be drawn from this study is, among others, the use of probiotics is believed to act as remedial agents of autism.

In my opinion, it is the way we live our life (lifestyle) that determines the proper balance of intestinal microbiota. In other words, taking probiotics alone as a primary intervention as an effort in reducing symptoms of ASD while ignoring to practice appropriate lifestyle especially by adopting dietary changes, may be futile.

Future Discoveries and Our Responsiblity

One of the most exciting scientific advances in recent years has been the acknowledgment that commensal microorganisms including intestinal microbiota are not basic "passengers" in our bodies but rather have key roles in our physiology, including our immune responses and digestion, and additionally in diseases. From time to time, new findings of microbiota roles in human health are emerging and reported in prestigious journals.

As a host, it is our responsibility to continually living symbiotically with them by taking care of our well-being via adopting a holistic approach in lifestyle. As an element of lifestyle, proper diet plays a vital role in maintaining the diversity of intestinal microbiota. What we consume also feeds off trillions of bacteria inhabit the intestines.

Two of the most widely studied elements in the field of gut microbiota are prebiotics and probiotics, both of which are beneficial for its homeostasis in term of diversity. As a healthy balance of microbiota positively impacts the digestive condition and various functions of the body, a varied and balanced diet are crucial.

Varied and Healthy Foods


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