The bacteria in the colon produce large amounts of vitamins B and K through fermentation. Vitamin K is a fat-soluble vitamin that helps blood clot and prevents excessive bleeding. Bacteria in the large intestine synthesize vitamins, not minerals, neither do they synthesize glucose but they do play a role in other processes that are important for the body.
Also, bacteria in the large intestine can synthesize amino acids. This process helps regulate the body's amino acid balance.