Bacteria that live inside the gut have great importance in digestion. Some polysaccharides like cellulose, hemicellulose and pectin are not digested by the digestive enzymes secreted by the body, instead, they are broken by enzymes released by bacteria of the gastrointestinal tract. The intestinal bacterial flora also make vital substances for the functioning of the bowels facilitating or blocking the absorption of nutrients and stimulating or reducing peristalsis. Some gut bacteria are the main source of vitamin K for the body and so they are essential for the blood clotting process.

In the intestinal flora there are utile but also potentially harmful bacteria. It is estimated that more than 100 trillion bacteria live in a human gut. Some bacteria are useful too because they compete with other species preventing excessive proliferation of these bacteria.   _{Your Comment}