A human being has about 10 trillion human cells, but there are also other tiny creatures living in the human body. These creatures are clusters of bacteria in different parts of the body, such as on the surface or in the deep layers of the skin, in the mouth and the nose, and in the lungs and the stomach. But the greatest number of bacteria, about 100 trillion that weigh up to 2 kg, reside in the gut. This may sound somewhat frightening, but these bacteria have a crucial and positive influence on our health.

The first scientist who paid attention to these microbes in the gut and their importance was Ilya Mechinikov, who won the Nobel Prize in 1908 for his studies on the immune system. He was living in Paris in the late 19th century when thousands of people were suffering from cholera, a disease that was sweeping the European continent. As a scientist, Mechinikov was naturally driven to study this disease, which eventually led him to eat food containing the cholera bacteria. Remarkably, he didn't get sick. Intrigued, he decided to increase his sample size and recruited a colleague to do the same. The colleague didn't get sick either. But when he tested on another colleague, the poor man became critically ill and almost died.

What made them react to the same disease in different ways? By studying cholera under the microscope, Mechinikov found that certain species of bacteria from the human gut supported and stimulated the cholera's growth, while other species prevented it. After further study, he realized that our gut bacteria were essential for human health, and that the right balance of microbes inside of us could help prevent disease. Sadly, this view was not widely accepted at the time and was eventually forgotten after Mechinikov died in 1916.

A decade later, antibiotics were discovered. Antibiotics have saved countless lives by giving doctors and scientists the means to kill harmful germs that infect wounds, cause deadly diseases, and spread through food. However, as time has passed, the miraculous power of antibiotics has caused some doctors to use them too much. Moreover, people have come to see all germs as threats to good health, and as a result, they have done their best to keep their bodies clean of them. A war has been waged on microbes, and people have spent almost a century trying to kill them. Unfortunately, this attitude has had harmful effects on health, resulting in a rise in allergies and immune deficiencies.

Fortunately, scientists are beginning to see the relationship between microbes and the human body in a new light. According to recent studies, thousands of species of microbes in the gut have different functions, and good health is incredibly dependent upon the well-being of these numerous and diverse microbes. By occupying our bodies first, they prevent harmful germs from growing there. Therefore, people with a healthy bacterial community may remain unaffected by slightly infected food, but those with fewer or unbalanced microbes may suffer from food poisoning. They also facilitate our immune system by distinguishing between good substances that should be allowed into our bodies and harmful ones that should be rejected. Therefore, having insufficient germs or losing important ones leads to various health problems.

Some microbes help the stomach digest some vegetables and milk. If they are absent, it is difficult to absorb certain vegetables and milk, which eventually leads to an unbalanced diet and poorer health. Other bacteria help the body produce vitamins B and K, as well as important amino acids.

Some microbes in the gut determine whether particular drugs are toxic to the liver and whether or not other drugs will work to combat a heart condition. Other microbes regulate the appetite hormone, which makes a person feel full or hungry. Losing these microbes may result in increased body fat. One recent study found that transplanting a lean mouse's gut bacteria into an overweight mouse's stomach made the overweight mouse lose weight. Similarly, doing the reverse made the lean mouse gain weight.

Some researchers who have realized the crucial role of microbes are trying to apply this knowledge to cure people. Instead of using antibiotics, they have found that introducing or increasing the necessary germs that the person lacks can cure some diseases. For example, there were people who had a terrible form of diarrhea, so they had to go to the bathroom up to 20 times a day. These people had not been helped by antibiotic therapy. Researchers transplanted some of the good microbes from a healthy donor into these patients' guts. One day after they received their transplant, all their symptoms cleared up and the diarrhea vanished. They were essentially healthy again. The bacterial community in their own guts came to resemble the donor's bacterial community.

Given this knowledge, nutritionists suggest the following ways to keep our guts healthy. One of the easiest ways to do this is through our food. Good microbes can be found in many of the foods we eat and whatever else gets to our mouths. By eating foods that contain beneficial bacteria, such as yogurt, gimchi, and natto, or choosing a diet packed with good, fresh nutrients, such as garlic, onions, and unprocessed grains rich in fiber, we can cultivate good microbes. Of course, it is also important to protect the germs that already exist in our bodies by avoiding processed food. Heavily processed foods such as frozen food, packaged snacks, and canned food not only lack beneficial bacteria but also contain chemicals that damage the good microbes in our guts.

Another way to increase beneficial microbes is to spend more time in nature. Places with diverse life forms, such as woods, beaches, parks, or even small farms and gardens, are abundant with diverse bacteria that germ-free urban environments lack. In addition, we should use antibiotics wisely; we must use them much less frequently. Although antibiotics play a crucial role in keeping us healthy by eliminating dangerous germs, they cannot tell good bacteria from bad ones. Therefore, whenever we use antibiotics, we inevitably damage beneficial bacteria as well as the harmful bacteria that we want to get rid of.

If we can affect microbes and they can affect our bodies, it makes little sense to talk about the two separately. Maybe it is time to stop thinking of microbes and ourselves as different beings. We need to start thinking of the microbes in our guts as a part of our bodies, like the blood cells and the neurons, and take care of them just as we take care of our skin and our teeth.