Seeing the wood for the trees: A new way to view the human intestinal microbiome and its connection with non-communicable disease.

Our paper briefly reviews the connection of the intestinal microbiome to the rise in non-communicable conditions related to atopic disease, obesity and mental health. We consider that the microbiome is best treated as if it were a single entity and have borrowed the terms semiochemical, allomone and kairomone (5) to describe interspecies relationship between the microbiome and ourselves (Fig. 1). We use the term dysbiosis to describe the breakdown of these relationships leading to disease (Fig. 2). As a result of this analysis we tentatively suggest that components of the microbiome assess microbial antigens in our food and pass this information back to our immune system via as yet undescribed chemical messengers: kairomones. We call these hypothetical microbial agents Sentinel Cells. Our suggestion is that atopic disease arises partly as a result of consuming processed food that has insufficient antigens to activate this kairomone feedback mechanism, which atrophies as a result. We note that this is potentially similar to the Old Friends concept of Rook and his co-workers (16). We suggest that obesity is a consequence of dysbiosis-induced waning of the output of allomone-like psychotropic compounds (including the known microbial metabolites dopamine and serotonin) leading to the weakening of the gut-brain axis and a negative effect on mental health. Although dysbiosis can occur in other ways, including antibiotic use and sterile caesarian section, we believe that all these problems can be overcome to provide a future free of these non-communicable diseases.