The role of host molecules in communication with the resident and pathogenic microbiota: A review

The human body harbours around 100 trillion microbial cells within a network of complex ecosytems. There is now a well-established correlation between microbes and health in conditions associated with chronic inflammation, and much research has focused on the microbe-to-human axis of communication. At a molecular level, the particular microbial activities and functions that dictate a “healthy” microbiota remain enigmatic. However, evidence from human and animal studies suggests that the host exercises control over the composition of microbial populations through the tightly regulated production and release of hormones, metabolites, nucleic acids, and immune effectors like cytokines. Many of these molecules are taken up by, or bind directly to, microbial cells, and it has been proposed that extracellular vesicles may help to carry these molecules directly to bacteria for enhanced uptake. Understanding how host control of the microbiota may become dysregulated in disease states will lead to novel ways of treating diseases and symptoms. Here we review the evidence for host regulation of the resident and pathogenic microbiota, with a focus on molecular mechanisms of communication in health and disease, and pinpoint how this knowledge may benefit future therapeutics.