Interactions between gut microorganisms and the role of their metabolites in host health

<p indent="0mm">Over the past few decades, significant progress has been made in understanding the potential relationships between gut microbes and health, and clarifying that the intestinal tract is an intact micro-ecosystem with highly complex interactions between gut microbes. At present, studies focusing specifically on the relevant relationships between gut microorganisms are still in their infancy, and only some simple one-to-one relationships, rather than networks of interactions between microorganisms, have been elucidated. The present review summarized the interactions between gut microorganisms, as well as the pathways by which they interact with the host via metabolites. Among the relationships between gut microorganisms, bacteria are one of the most studied microorganisms in the ecology of intestinal tract communities, and the main interactions among gut bacteria are neutral, symbiotic, competitive, and predatory. Neutral or beneficial interactions can be defined as symbiosis, and mutualistic symbiosis is a special mechanism of microbial interactions in which some microorganisms are able to provide each other with nutrients or survival conditions needed by the other and thus mutually benefit, e.g. <italic>E</italic>. <italic>coli</italic> in the gut ferment glucose to excrete organic acids to provide a carbon source for <italic>R</italic>. <italic>palustris</italic>, while <italic>R</italic>. <italic>palustris</italic> converts N₂ to NH₄⁺ to provide an essential nitrogen source for <italic>E</italic>. <italic>coli</italic>. Resistance interactions (including competition, mutualistic symbiosis and predation) also occur among gut bacteria, where they compete for nutrients and ecological niches, produce inhibitory substances (e.g. antibiotic bacteriocins) or invade host cells to cause host death and thereby exert an inhibitory effect on other bacteria. <italic>Candida</italic> and <italic>Saccharomyces</italic> dominate the human gut fungal community, and the relationships between the gut fungal and bacteria are mainly cooperative and antagonistic, e.g. <italic>C</italic>. <italic>tropicalis</italic> can form biofilms with pathogenic bacteria (such as <italic>E</italic>. <italic>coli</italic> and <italic>S</italic>. <italic>marcescens</italic>) to protect them from immune cells, and <italic>L</italic>. <italic>plantarum</italic> can inhibit <italic>C</italic>. <italic>albicans</italic> abundance in the gut by producing beneficial metabolites. The relationship between gut bacteria and phages is parasitic and can be characterized by “kill the winner” kinetics and the “red queen hypothesis” depending on the type of phage. The relationship between host and gut microbes is mainly mutualistic, with the host providing a favourable ecological environment for the growth of gut microbes and the gut microbes contributing to the health of the host through a variety of metabolic responses. In the process of microbial regulation of organismal health, microbial metabolites are the main medium. Key metabolites, such as SCFAs, secondary bile acids and aromatic metabolites, can stimulate corresponding immune cells, and regulate the intestinal barrier effect via intestinal receptors and transport proteins, and can also dialogue directly with the host via the circulation, ultimately playing an important role in the host’s nutrient absorption, metabolism, immune system development and pathogen defense. Overall, the regulation of health by gut microbes should be the result of a network of interactions, rather than changes in the levels of individual microbes in the gut. The interactions between gut microbes and their potential relationship with functional metabolites and health should be further explored by combining culture genomics and artificial intelligence.