Conductors in the Gut that tune the Brain: The Microbiome story

In popular perception, micro organisms, especially bacteria and viruses, are often linked to disease. This kind of perception is definitely not without a reason. Pathogenic agents have been responsible for so many deaths over the years that diseases like the plague have shaped the course of human history.

But do you know microbes are actually necessary for us to stay healthy, that there are 100 times more microbial genes in our body than our own? Do you know what happens when the microbial population in our body is disturbed by for example, use of antibiotics?

Our bodies are made of organs, organs of tissues and tissues of cells. From some recent estimates for every 10 human cells, there are about 13 microbial cells in our body. These microbes are present in almost every organ of the body and include of fungi, viruses and, predominantly, bacteria. The total microbial population living in our bodies is called the microbiota and the total collection of genes (genes are the parts of DNA in a cell that have the code to make proteins) genes from the microbiota is called the microbiome. There are about 20,000 to 30,000 unique human genes in our body. By comparison the size of human microbiome is estimated to be about 30 lakh unique genes. That is, our body has 100 times as many microbial genes as human ones. A healthy human body is a combination of the human cells and the microbial cells. Any disturbance in this balance is called disbiosis. It is the disbiosis that results in the many pathogenic diseases.

The microbiome at each site in the body is different, specific and diverse. The composition of the microbial variety in each part of the body is it’s signature and the microbial signature of no two individuals is similar, not even between identical twins. So just like the fingerprints, microbiome can be used as a biometric to identify a person. And because the microbiome after death changes as the body decomposes, it can also be used to determine the time of death of a person.

But how do we get the microbiome in our body?

We acquire microbiome for the first time during birth. As the fetus passes through the birth canal, some of the microbes in the mother’s vagina are acquired. Babies born through C-section, however, have their initial microbiome exposure from the mother’s skin. There have been some studies that suggested that this difference in the microbiome acquired at the time of birth could be a reason for greater susceptibility of C-section babies to ailments like childhood type 1 diabetes.

Immediately following birth, the microbiome changes rapidly. During breast feeding, microbes from the mother’s skin become part of the baby’s microbiome. As the baby starts consuming  different types of food, the microbiome in the gut also changes. So too with changes in the environment. The microbiome generally stabilizes at about 2.5 to 3 years after birth. That however doesn’t mean it’ll remain the same for the rest of a person’s life. It does change with changes in diet (for example someone becomes from non-meat eater to meat eater or vice versa), environment (for example if a person moves from a warm place to a cooler place), etc. Probably the biggest contributor to changes in microbiome is the use of medicines, especially antibiotics. Frequent and indiscriminate use of antibiotics causes long-term changes in the gut microbiome composition.

What does microbiome do? Why is a healthy microbiome required?

There are many types of food material that we consume but do not have the enzymes in our body to digest, like starch or fibrous material in vegetables or fruits. The gut microbiome provides the enzymes required to digest these. In one study, it was observed that the microbiome of subjects from Japan contained the gene required to digest Prophyran (a type of carbohydrate found in red algae). But the gene was not observed among individuals from US. This is because the sea weed is a common part of the diet in Japan but not in the US. It would not be an exaggeration to say that if not for the gut microbes, our food choices will be severely restricted.

Digestion of fibers by the microbiota helps us in other ways also. Fermentation of the dietary fiber by the enzymes of gut microbes releases molecules called short chain fatty acids (SCFA) leading the colon to become more acidic. This prevents non-native, harmful bacteria from colonizing the gut and causing disease. The SCFAs also play a role in the activation of immune system, maintaining blood sugar and modulating mental health. 

It has also been observed that the gut microbiome plays a role in the development of obesity. When microbiomes from a pair of twins, one normal and the other obese, were transplanted into mice, those receiving the microbiome from the obese brother developed obesity while the mice receiving the microbiome from the normal twin did not. 

Another aspect where the microbiome plays an important role in our body is in the mental health. Mice grown right from birth in totally sterile conditions do not have any microbiome. These are called germ free (GF) mice. When GF mice were put under stress, they had more elevated levels of stress hormones than normal mice. This elevated stress response was reversed when the GF mice were transplanted with bacteria that are generally found in new-born mice. In another study, researchers found that oral administration of Lactobacillus, a bacteria commonly found in fermented foods like curd, was orally administered, stress-induced anxiety or depression like symptoms were reduced in mice. Reduced or absent gut microbiota not only results in behavioral changes but also biochemical changes in the brain such as in the level of neurotransmitters like serotonin that is involved in modulation of stress and anxiety. 

What can we do to maintain a healthy microbiome?

Most obvious is to avoid unnecessary and frequent use of antibiotics. Any drastic and sudden changes to the regular diet should also be avoided; changes should be measured and gradual. Including probiotic and prebiotic food items in the diet will also help in maintaining a healthy microbiome. Prebiotics are fibre rich  food materials like leafy vegetable, grains, pulses, etc. Probiotics are food items like curd which contain live bacteria (eg. Lactobacillus) which help in replenishing the gut microbiome. 

To conclude,

  • Microbes are not always harmful or disease causing
  • Human body has more microbial cells than human cells
  • It is the disturbance in the natural microbiome composition that results in diseases.
  • Maintaining a healthy microbiome is neccessary for a healthy body.

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