Approaches to dissect the vitamin biosynthetic network of the gut microbiota.

B-group vitamins and vitamin K are essential micronutrients required for numerous cellular processes in both microbial and human physiology. While traditionally considered to originate predominantly from dietary sources, the biosynthetic capacity of the human gut microbiota has recently been recognized as a valuable, though historically underappreciated, endogenous source of these vitamins. In particular, the microbial contribution to the host vitamin pool is increasingly acknowledged as a functionally relevant aspect of vitamin homeostasis, especially in the colon, where microbiota-derived vitamins may be absorbed via specific transport mechanisms. This review provides a comprehensive overview of our current understanding of the biosynthesis of B-group vitamins and vitamin K by human gut-associated bacteria, with particular emphasis on key methodologies employed to assess if, how and to what extent members of the gut microbiota supply their host with such micronutrients. Through an integrated overview of available evidence, we highlight both the progress made and the outstanding challenges in elucidating the microbial contribution to the host vitamin metabolism.