which vitamin is produced by bacteria in the colon
Vitamin B12 B12 is produced in nature only by some prokaryotes (certain bacteria and archaea); it is not made by any multicellular or single-celled eukaryotes.[31][32] It is synthesized by some gut bacteria in humans and other animals, but humans cannot absorb the B12 made in their guts, as it is made in the colon which is too far from the small intestine, where absorption of B12 occurs.[33] Ruminants, such as cows and sheep, absorb B12 produced by bacteria in their guts.[33] For gut bacteria of ruminants to produce B12 the animal must consume sufficient amounts of cobalt.[34]