What are the main functions of the bacterial flora within the human gut?

Bacteria that live inside the gut have great significance 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 free by bacteria of the gastrointestinal tract. The intestinal bacterial flora also makes vital substances for the functioning of the bowels facilitating or blocking the absorption of nutrients and stimulating or decreasing peristalsis. Some gut bacteria are the main source of vitamin K for the body and so they are necessary for the blood clotting process.