The microbiome is the collective name for the trillions of bacteria and other microorganisms that normally live peacefully in and on humans (the skin and the gut are major reservoirs), and many of which, through their rich array of genes , perform functions that support our existence in diverse ways. There are said to be 100 trillion bacteria residing in the gut alone. Some of them help humans digest the sugars in cow’s milk; there are others that break down dietary fiber and, in the process, re
What Do Those Billions of Bacteria Do?
A major function of the gut microbiome is to protect you from disease-causing bacteria that try to hitchhike their way into your system on food. Starting with your mouth, the digestive system is a major portal through which pathogens enter the human body. Perhaps you’ve even received a phone call from your local supermarket advising you not to eat those onions you bought, because they may be tainted with the food-poisoning pathogen Salmonella . The resident bacteria in your gut are the first line of defense against such infection (after your vigilant grocer).
It is because of beneficial bugs in the gut that we can get some extraordinary substances from foods. Of the hordes of bacteria in the digestive tract, very few of them live in the acid environment of the stomach, which breaks down food and prepares it for digestion. A small number of them reside in the small intestine, where much of food digestion takes place. The vast majority of gut bacteria inhabit the large intestines, the colon.
There, they tackle foods that are resistant to digestive enzymes—notably foods rich in the complex carbohydrates known as fiber. From them, they create micronutrients that boost health in ways that are still under study. They also aid in the digestion of protein and, to some extent, fat. In the process of digesting foods, gut bacteria release a great array of molecules known as metabolites that have effects on virtually every system of the body. The actions of the microbiome extend well beyond the gut.
Gut bacteria play a significant role in stimulating the growth of the immune system and regulating its activity. One way they do it is by maintaining the integrity of the cells lining the gut; they prevent escape into the bloodstream of any microbes or toxic substances that might otherwise leak out and cause chronic systemic inflammation, a contributor to disorders ranging from inflammatory bowel disease to depression . Dysfunction of the gut barrier, a condition known as “leaky gut,” is linked to autism in ways still not understood.
The Composition of the Microbiome
There are thought to be somewhere between 1,000 and 1,500 species of bacteria residing in the gut. The composition of the microbiome varies somewhat from person to person, but in each individual is relatively stable. That, however, does not mean it is static; it changes in response to what we do, where we are, and, especially, what we eat.
For example, engaging in physical activity has been found to favorably alter the composition of the microbiome. Stress disturbs the balance of bacteria in the gut. Taking medications, especially antibiotics, and especially during childhood , kills off many beneficial bacteria in the gut, and while they recover slowly, some may never return.
Bacterial diversity is considered a hallmark of wellness, important for maintaining a generally healthy physiological state. It is associated with many health-related measures—positively associated with height, physical activity, fruit and vegetable consumption (especially the cabbage family), even caffeine consumption.
The most abundant bacteria in a healthy gut belong to a family called Firmicutes ; they have been shown to make up about 80 percent of a well-balanced microbiome. Another prominent family of bugs in the microbiome is Bacteroidetes , constituting about 15 percent of a balanced microbiome. The relative proportion of Bacteroidetes to Firmicutes is thought to drive gut bacterial diversity.
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