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The Microbial Menagerie in Your GI Tract: Friends or Foes?
John Y. Kao, M. D, is Assistant Professor, Nirmal Kaur, M.D., is Fellow and Vincent B. Young, M.D., Ph.D., is Assistant Professor, Department of Internal Medicine, Infectious Diseases Division, Department of Microbiology & Immunology, University of Michigan Medical School, Ann Arbor, Michigan.
It’s a subject that many of us are not typically wildly excited to discuss: our gastrointestinal system, or GI tracts. The interworkings of our bowels present somewhat of a mystery to most, and talking about them and understanding just how they function can make people feel uneasy, queasy, or just plain grossed out. However, the functioning of our GI systems is central to our health and well-being, and the complexity of their operation is actually rather astounding. Not only is the system responsible for absorbing the vital nutrients that we eat to nourish our bodies, but it also has the hefty task of ridding the body of all the things that we don’t want in it, like waste matter and harmful toxins.
The GI tract technically consists of everything from where food enters your body to where it exits. So the GI tract is made up of the mouth, esophagus, stomach, small intestine, large intestine (including colon and rectum), and anus. Food is moved through the digestive system with a rhythmic, wavelike motion called peristalsis. To coordinate the many actions and responsibilities of the GI tract, a nervous system, known as the enteric nervous system controls how the gut works. In fact, there are as many neurons on the enteric nervous system as there are in our spinal cords.[A]
But even given this complexity, there’s still more to the story than just this. Many people may be aware that when it comes to our GI tracts "we’re not alone", so to speak. There are billions and billions of organisms, known as microbes, living in our GI system. Once upon a time, the medical community assumed that these tiny organisms were just hitching a ride on unsuspecting hosts (us), without actually contributing anything to our general well-being.
Now, however, there’s overwhelming evidence to suggest that these microorganisms are actually extremely beneficial, and that they play key roles both in maintaining the health of our GI systems (and, in many ways, the rest of our bodies) and in the development of disorders and diseases of the GI tract.
Under normal conditions, these microbes interact with and affect the cellular lining of the GI tract as well as with the immune systems of the human (or animal) in which they’re living. The relationship, known as "symbiosis", is complicated one — but in its normal state, symbiosis provides a benefit to all parties involved. But if the normal balance is upset for any reason, disease can result. This article will go over how microbes in the gut function overall, and what can occur if the natural balance of these interesting organisms gets out of whack.
Just What Do The Microbes of the Gut Do?
The number of microorganisms living in the intestines is staggering. There aren’t just a few different types, but rather there are at least 10,000 different species of microorganisms that dwell in the gut.(1) Researchers have had to design clever methods (mostly using complicated RNA and DNA techniques) to determine the likely number of these organisms, because many of them won’t grow in culture, which makes them difficult to study.(1)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)
A variety of factors affects the balance of species, like a person’s nutrition, one’s physiology in general, his or her immune system and general health. The makeup of the population of microbes can vary quite a lot among people and even within a single person over a period of time, as the factors mentioned above can change considerably throughout the years.
The Human Microbiome Project is a large and ongoing set of studies to understand exactly how these little critters affect human health and well-being.(2)
The variety of functions that microbes play is almost as impressive as their number.(3)(4)(5)(6)(7)(8)(9)(10) Functions range from helping us metabolize nutrients in the food we eat to helping the gut develop. What’s more, changes in the makeup of the microorganism population are related to obesity, inflammatory bowel disease (IBD), and diarrhea that may be a side-effect of antibiotic use. All these changes that we feel on our end are thought to be related to changes in the metabolic activity of the microorganisms in our guts, or how they interact with our immune systems. All these reasons are why the makeup of the microbe population can have such strong effects on our overall health.
Changes in Microbe Makeup Can Lead to Intestinal Diseases and Distress
Since it is clear that the microbes that inhabit our G.I. tract can help keep us healthy, disturbances in this complex community can lead to a variety of illnesses. In some ways, this is quite different from what we think of when we consider how microbes might make us ill. We are used to thinking about a microbe causing an "infection" by invading our body, but it’s a different way of thinking to consider that the lack of particular microbes or perhaps a change in the community of microbes can make us sick. In the following sections we will discuss a number of illnesses where changes in the G.I. microbes are thought to contribute to the disease process.
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