Vitamin B-12 sources and microbial interaction

Vitamin B-12 is synthesized only by certain bacteria and archaeon, but not by plants. The synthesized vitamin B-12 is transferred and accumulates in animal tissues, which can occur in certain plant and mushroom species through microbial interaction. In particular, the meat and milk of herbivorous ruminant animals (e.g. cattle and sheep) are good sources of vitamin B-12 for humans. Ruminants acquire vitamin B-12, which is considered an essential nutrient, through a symbiotic relationship with the bacteria present in their stomachs. In aquatic environments, most phytoplankton acquire vitamin B-12 through a symbiotic relationship with bacteria, and they become food for larval fish and bivalves. Edible plants and mushrooms rarely contain a considerable amount of vitamin B-12, mainly due to concomitant bacteria in soil and/or their aerial surfaces. Thus, humans acquire vitamin B-12 formed by microbial interaction via mainly ruminants and fish (or shellfish) as food sources. In this review, up-to-date information on vitamin B-12 sources and bioavailability are also discussed.