The elucidation of the mechanisms by which the gut microbiota can influence host physiology requires that the microbiota be examined in an environment which simulates as closely as possible the in vivo milieu. To this end we have designed a simulated small intestinal medium (sSIM) that is based on the in vitro intestinal model systems used in the pharmaceutical industry to assess drug stability. An advantage of the sSIM methodology is that food sources consumed by animals ranging from farm production animals to humans can be used in which the first 3 phases of digestion (oral, gastric, and proximal intestinal) are replicated. The end product is a medium that contains biochemical precursors not found in traditional microbiological media that may be used for discovery of novel microbial-produced products following the introduction of gut microbiota. We have employed sSIM medium in such a fashion to examine the ability of gut microbiota to produce neurochemicals that are primary microbial endocrinology-based mediators of the microbiota-gut-brain axis. Microbial endocrinology represents the union of the fields of microbiology and neurobiology and seeks to identify the mechanisms by microbiota influence host physiology and behavior through the recognition and utilization of neurochemicals. While the capacity for bacteria to produce neurochemicals has been recognized for decades, the degree to which this may occur in the environment of the gut has been poorly understood. Utilizing sSIM medium produced from mouse chow, we have discovered that the gut microbiota is capable of the production of the neurochemical dopamine. Within the host gut, dopamine participates in neuronal and immune functioning and is critical to the generation of immune responses. Further, a normal constituent of the gut, Enterococcus faecium, has been shown to be a potent producer of dopamine. The discovery of a bacterium that previously had not been recognized either through standard microbial culture or bioinformatic analyses to be capable of the production of a neurochemical critical to maintenance of host physiology provides solid evidence that the use of physiologically-relevant media with which to explore the capacity of the gut microbiota to influence gut health and behavior is needed. The prototypical design of the sSIM medium also allows for different nutritional inputs to be evaluated such that different diets can be examined for their ability to influence gut microbial activity.

Use and relevance of microbial endocrinology in the design of in vitro gut models for understanding nutrient-microbiome interactions

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ACS Fall 2022

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