Review: Bacterially produced vitamin K2 and its potential to generate health benefits in humans

Several bacterial species associated with the human gut and with fermented foods produce menaquinones (MK-n; vitamin K2) as part of their energy metabolism. Given that MK-n offer multiple health benefits to humans, research into this topic is of increasing interest. To outline the current understanding of bacterial MK-n production and how it relates to the gut microbiota, food fermentation, the production of dietary supplements, and health. In bacteria, MK-n display various functions in addition to their chief role in energy production, including a role in defense mechanisms, the sensing of environmental changes, the reduction of metals, sporulation, and pathogenicity. In humans, they are relevant for health beyond the effects of phylloquinones (vitamin K1), which are primarily related to blood coagulation, as they seem to be more effective extrahepatically, impacting on cardiovascular and bone health and protecting against non-communicable diseases. While the contribution of gut microbiota to the human vitamin K budget remains unclear, more is known about fermented foods. Some fermented foods from either plant (e.g., sauerkraut and natto) or animal origin (e.g., cheese) are valuable sources of MK-n, even compared to non-fermented animal source foods, which only contain limited amounts of one specific form (MK-4). Bacillus subtilis, commonly used in the production of natto, is the most employed microorganism at industrial level for the production of dietary MK-7 supplements, a process which can be optimized by selecting the appropriate fermentation parameters.