Intestinal microbes boost our immune system and stop infectious diseases

     These days, interest in intestinal microbes is steadily growing. Already, more than 30 years have passed since such microscopic organisms were found to be related to better health. For example, it is now common knowledge that a daily helping of yogurt nurtures the bifidobacteria and lactic acid bacteria in our intestines which boosts our health. Here, we take a look at the physiological benefits of intestinal microbes, from historical beginnings to the latest research.

Dr. Tomotari Mitsuoka, Emeritus professor of Tokyo University, has published a review of advancements in the research field of intestinal microbes (“The Progress of Intestinal Microbiota Research”, T. Mitusoka, (in Japanese) Intestinal Bacteria Research Journal(2011) 25, p113). According to this review, around 1850 in France, Louis Pasteur first recognized microbes as living organisms, which had been first seen only by microscope. Yeast used for wine production was also identified as one of these microbes. He established a technique to sterilize agar plates of bacteria in order to isolate and identify colonies of a single bacterial species. Roberto Koch and his colleagues found that various bacteria including cholera and dysentery germs lead to infectious diseases around the year 1900. Since then, much attention has been paid to infectious bacteria. In 1907, Ilya Mechinikov found that lactic acid bacterium in yogurt supports longevity. In 1960 Dr. T Mitsuoka found that intestinal bacteria are divided into 2 groups; one is good for our health including lactic acid bacteria and bifidobacteria, and the other is bad for our health including toxic colon bacilli, and Clostridium perfringens (Intestinal bacteria list below).

Fig. A schematic process of intestinal infection of bacteria baesd on our unhealthy condition

  “Good” intestinal germs can aid digestion of dietary fibers mainly from vegetables, thus increasing the nutritional value of such foods which otherwise might not be broken down by our intestinal enzymes. The digested products of dietary fibers are mainly short chain fatty acids (SCFA’s) like lactic acid, acetic acid and propionic acid. These fatty acids are excreted to the outside intestine through a specific area of the intestinal surface layer. Near the outer intestine, white blood cells await as part of our immune response, ready to remove unwanted foreign materials from the foods passing through the intestinal tract.

Roughly 70% of all immune cells are gathered around the intestine. The short chain fatty acids excreted from the intestine activates and/or changes the function of these immune cells (Gut microbiome and Regulation of mucosal immunity, Hideki IIjima and Hiroshi Kiyono, (in Japanese) Intestinal Bacteria Research Journal (2021), 35, p205). When “bad” intestinal germs proliferate in our intestine, they excrete poisonous toxins leading to unfavorable alkaline conditions in the intestine and even to serious intestinal inflammation like inflammatory bowel disease.

    In 2015 NHK (Japan Public Broadcasting Corp.) published a book reporting the latest research progress on the newly identified physiological functions of SCFA’s. For example, such acids affect the locus of fatty acid accumulation known as adipocyte cells, and also reduce cellular uptake of sugars, an effect which reduces obesity. SCFA’s also affect parasympathetic neurons to induce relaxation. It is also known that T-reg cells, a type of white blood cell, are activated by the SCFA’s which reduces the accumulation of T cells that may contribute to intestinal inflammation.    Further detailed study is ongoing to determine the precise mechanism of fatty acid functions. On the surface of intestinal epithelium cells, a sugar molecule named Fucose is excreted. SCFA’s increase production of this sugar, which stimulates the formation of a viscous mucous layer on the intestine surface that prevents bacterial infection. This sugar also serves as a source of nutrition for good germs in the intestine. Research results reported by Y. Goto et al. (Y.Goto, et al. Innate lymphoid cells regulate intestinal epithelium cell glycosylation, Science (2014) 345, 1254009) indicate that intestinal bacteria, immune cells and intestinal epithelium cells collaborate to ward off microbial infection.

Fig. Short chain fatty acid plays an important role for our immune system and intestinal infection of bacteria

   Any summary of the importance of intestinal bacteria should note that rather than a single species of intestinal bacteria, a mix of microbial multitudes is responsible for boosting our immune system by maintaining a healthy intestinal environment. In fact, a diversity of intestinal bacteria from a healthy human body is needed to cure several illnesses. The physiological roles of intestinal bacteria with their metabolic effects appear to be an essential organ of our body.