No abstract

Elements of the human gut microbiota metabolise many host- and diet-derived, non-digestible carbohydrates (NDCs). Intestinal fermentation of NDCs salvages energy and resources for the host and generates beneficial metabolites, such as short chain fatty acids, which contribute to host health. The development of functional NDCs that support the growth and/or metabolic activity of specific beneficial gut bacteria, is desirable, but dependent on an in-depth understanding of the pathways of carbohydrate fermentation. The purpose of this review is to provide an appraisal of what is known about the roles of, and interactions between, Bacteroides and Bifidobacterium as key members involved in NDC utilisation. Bacteroides is considered an important primary degrader of complex NDCs, thereby generating oligosaccharides, which in turn can be fermented by secondary degraders. In this review, we will therefore focus on Bacteroides as an NDC-degrading specialist and Bifidobacterium as an important and purported probiotic representative of secondary degraders. We will describe cross-feeding interactions between members of these two genera. We note that there are limited studies exploring the interactions between Bacteroides and Bifidobacterium, specifically concerning β-glucan and arabinoxylan metabolism. This review therefore summarises the roles of these organisms in the breakdown of dietary fibre and the molecular mechanisms and interactions involved. Finally, it also highlights the need for further research into the phenomenon of cross-feeding between these organisms for an improved understanding of these cross-feeding mechanisms to guide the rational development of prebiotics to support host health or to prevent or combat disease associated with microbial dysbiosis.

Tyrosinase starts melanogenesis and determines its course, catalyzing the oxidation by molecular oxygen of tyrosine to dopa, and that of dopa to dopaquinone. Then, nonenzymatic coupling reactions lead to dopachrome, which evolves toward melanin. Recently, it has been reported that D-tyrosine acts as tyrosinase inhibitor and depigmenting agent. The action of tyrosinase on the enantiomers of tyrosine (L-tyrosine and D-tyrosine) and dopa (L-dopa and D-dopa) was studied for the first time focusing on quantitative transient phase kinetics. Post-steady-state transient phase studies revealed that L-dopachrome is formed more rapidly than D-dopachrome. This is due to the lower values of Michaelis constants for L-enantiomers than for D-enantiomers, although the maximum rates are equal for both enantiomers. A deeper analysis of the inter-steady-state transient phase of monophenols demonstrated that the enantiomer D-tyrosine causes a longer lag period and a lower steady-state rate, than L-tyrosine at the same concentration. Therefore, D-melanogenesis from D-tyrosine occurs more slowly than does L-melanogenesis from L-tyrosine, which suggests the apparent inhibition of melanin biosynthesis by D-tyrosine. As conclusion, D-tyrosine acts as a real substrate of tyrosinase, with low catalytic efficiency and, therefore, delays the formation of D-melanin.

Dietary manipulation of the HGM requires knowledge of how glycans available to this ecosystem are metabolized. The variable structures that decorate the core component of plant AGPs may influence their utilization by specific organisms within the HGM.

The human gut microbiota (HGM) is comprised of a very complex network of microorganisms, which interact with the host thereby impacting on host health and well-being. β-glucan has been established as a dietary polysaccharide supporting growth of particular gut-associated bacteria, including members of the genera Bacteroides and Bifidobacterium, the latter considered to represent beneficial or probiotic bacteria. However, the exact mechanism underpinning β-glucan metabolism by gut commensals is not fully understood. We show that mycoprotein represents an excellent source for β-glucan, which is consumed by certain Bacteroides species as primary degraders, such as Bacteroides cellulosilyticus WH2. The latter bacterium employs two extracellular, endo-acting enzymes, belonging to glycoside hydrolase families 30 and 157, to degrade mycoprotein-derived β-glucan, thereby releasing oligosaccharides into the growth medium. These released oligosaccharides can in turn be utilized by other gut microbes, such as Bifidobacterium and Lactiplantibacillus, which thus act as secondary degraders. We used a cross-feeding approach to track how both species are able to grow in co-culture.