Metabolism of Dietary Substrates by Intestinal Bacteria and Consequences for the Host Intestine

Metabolism of alimentary compounds by the intestinal microbiota and consequences for gut health

“…In the colon, the intestinal bacteria degrade proteins and peptides through their protease and peptidase activities, thus releasing amino acids [9]. Then, since colonocytes cannot absorb amino acids to any significant extent [10], the amino acids are used by the microbiota in anabolic and catabolic pathways, producing notably numerous bacterial metabolites including ammonia, phenolic and indolic compounds, hydrogen sulfide, polyamines, short-and branched-chain fatty acids, as well as various organic acids [3] (Figure 1). Regarding the indigestible carbohydrates, these alimentary compounds which include resistant starch, non-starch polysaccharides (mainly soluble and insoluble fibers), and indigestible oligosaccharides, are transferred to the large intestine where they are used as primary carbon and energy source for bacterial growth [11].…”

“…In the small intestine, low molecular weight polyphenols are partially absorbed by the enterocytes, but in contrast, the high molecular weight tannins are not absorbed in the small intestine and are thus entirely transferred to the large intestine [16]. The metabolic activity of the large intestinal microbiota towards both low molecular weight polyphenols and high molecular weight tannins releases numerous bacterial metabolites that are often organic acids and hydroxylated forms of polyphenols [3] and which include notably 3,4-dihydroxyphenylacetic acid (DOPAC), 3,4-dihydroxybenzoic acid (also called protocatechuic acid (PCA)), and 4-hydroxyphenylacetate (HPA) (Figure 1). These hydroxylated polyphenols have been studied for their action in terms of microbial communication and effects on the intestinal mucosa, as will be detailed in the subsections 2 and 3.…”

“…Bacteria depend on the substrates provided by the host for their growth and physiology. These substrates are from alimentary and endogenous origins [3]. The mammalian gastrointestinal tract includes the small and the large intestines.…”

“…The substrates from alimentary origin are partly different in the small and in the large intestine. In the small intestine, there are evidence that bacteria use a minor part of substrates originating from the alimentation for their metabolism and physiology, while in the large intestine, bacteria use mainly substrates that have not been digested (or not fully digested) in the small intestine, thus being transferred through the ileocaecal junction [3]. Bacteria use the available substrates in anabolic and catabolic pathways, allowing synthesis of macromolecules and diverse compounds that they need for growth, signaling, and physiological functions, as well as for energy production, while producing myriad of compounds during catabolism [3].…”

“…In the small intestine, there are evidence that bacteria use a minor part of substrates originating from the alimentation for their metabolism and physiology, while in the large intestine, bacteria use mainly substrates that have not been digested (or not fully digested) in the small intestine, thus being transferred through the ileocaecal junction [3]. Bacteria use the available substrates in anabolic and catabolic pathways, allowing synthesis of macromolecules and diverse compounds that they need for growth, signaling, and physiological functions, as well as for energy production, while producing myriad of compounds during catabolism [3]. The aim of the present review is to give illustrative examples of metabolic and intermediary products originating from alimentary compounds that are implicated in communication between microbes and in communication between microbiota and the intestine of the lodging host.…”