What Are Gut Bacteria?

Written by

Kara Rogers

Fact-checked by

Britannica Editors

History

Gut bacteria are microorganisms that naturally inhabit the digestive tract. They serve critical roles in the human body, particularly in the processes of digestion, vitamin synthesis and the synthesis of other beneficial compounds, immune regulation, and the prevention of colonization by harmful organisms. In addition to bacteria, which are most abundant, the gut is also home to archaea, fungi, viruses, and other agents. Together, these entities form the gut microbiota, the collective genetic material of which is known as the gut microbiome.

Composition

The human digestive tract contains trillions of microorganisms, the vast majority of which reside in the large intestine. This enormous population includes hundreds of different species of bacteria, with the greatest diversity belonging to the phyla Bacillota (Firmicutes) and Bacteroidota (Bacteroidetes), along with smaller numbers of Actinomycetota, Pseudomonadota, and Verrucomicrobiota. Which organisms are present in an individual’s gut is dictated by many factors, particularly age, diet, medications, and lifestyle.

Function

Gut bacteria carry out various functions, some of which the human body is unable to accomplish on its own. As an example, certain members of the gut microbiota ferment complex carbohydrates and dietary fiber; these nutrients escape digestion in the small intestine. The bacterial fermentation process generates short-chain fatty acids—among them acetate, butyrate, and propionate—that provide energy for cells in the colon and that help reinforce the intestinal barrier, as well as mediate inflammation systemic metabolism. The synthesis of certain vitamins, including vitamin K and several B vitamins, and the break down of bile acids, cholesterol, and other dietary compounds, is also facilitated by gut bacteria.

The gut microbiota also plays a central role in the development and function of the immune system. From infancy onward interactions between intestinal microbes and immune cells help the body distinguish harmless microorganisms from disease-causing pathogens and regulate inflammatory responses. Beneficial bacteria also provide colonization resistance, competing with harmful microbes for nutrients and attachment sites while producing substances that inhibit their growth. This protective effect helps reduce the risk of infection by pathogens such as Clostridioides difficile and Salmonella.

What kills gut bacteria?

The particular types of bacteria that constitute the gut microbiota may be altered by the use of antibiotics, which potentially eliminate bacteria that are helpful, in addition to those that cause disease. A diet low in fiber and rich in highly processed foods, as well as gastrointestinal infections, the use of certain medications (such as proton pump inhibitors), or chronic illness, aging, and prolonged stress, further influence the composition of the gut microbiota. Notably, these various factors can cause a decrease in microbial diversity or potentially disrupt the microbial community’s balance—a condition known as dysbiosis. Thus, while often resilient and typically able to recover over time, the gut microbiota is affected by repeated or prolonged disturbances, leading to digestive disorders and chronic diseases.

What increases gut bacteria?

Certain dietary and lifestyle habits help support a diverse gut microbiota that promotes health. Fruits, vegetables, legumes, whole grains, nuts, and other fiber-containing foods, for example, provide nutrients that support beneficial bacteria. Likewise, fermented foods such as yogurt, kefir, kimchi, sauerkraut, and miso, as well as prebiotics—nondigestible carbohydrates that nourish beneficial microbes—and probiotics, which contain live microorganisms, may also support gut health in some individuals. Regular physical activity, adequate sleep, and avoiding unnecessary antibiotic use further help maintain a balanced microbial community.