Carotenoid productivity in human intestinal bacteria and with functional analysis of their carotenoid biosynthesis genes.

The human intestinal microbiota that comprise over 1,000 species thrive in dark and anaerobic environments. They are recognized for the production of diverse low-molecular-weight metabolites crucial to human health and diseases. Carotenoids, low-molecular-weight pigments known for their antioxidative activity, are delivered to humans through oral intake. However, it remains unclear whether human intestinal bacteria biosynthesize carotenoids as part of the microbiota. In this study, we investigated carotenoid synthesis genes in various human gut and probiotic bacteria. As a result, novel candidates, the and genes, were identified in the carbon monoxide-utilizing gut anaerobe and the lactic acid bacterium subsp. . These gene candidates were isolated, introduced into , which synthesized a carotenoid substrate, and cultured aerobically. Structural analysis of the resulting carotenoids revealed that the and gene candidates of and L. mediate the production of 4,4'-diaponeurosporene through 15--4,4'-diapophytoene. Evaluation of the -homologous genes in these bacteria indicated their non-functionality for C-carotenoid production. and L. , along with the known carotenogenic lactic acid bacterium , were observed to produce no carotenoids under strictly anaerobic conditions. The two lactic acid bacteria synthesized detectable levels of 4,4'-diaponeurosporene under semi-aerobic conditions. The findings highlight that the obligate anaerobe retains aerobically functional C-carotenoid biosynthesis genes, potentially with no immediate self-utility, suggesting an evolutionary direction in carotenoid biosynthesis. (229 words).