Bacteria need “sleep” too? Microbiome circadian rhythmicity, metabolic disease, and beyond

Humans contain an “organ” composed of two to six pounds of bacteria, primarily residing in the gastrointestinal tract, which has gone largely unappreciated until recently. The gastrointestinal microbiome is a dynamic “organ” that performs important physiological functions for its host and can also be a factor in disease. Currently, animal models of germ-free mice, zebrafish, and fruit flies are utilized to address cause and effect relationships between resident microbes and their host. Like almost all living organisms, the gut microbiota regulate many of their changes in cellular activity and behavior over 24-hour cycles known as the circadian clock or rhythm. The microbiome has been shown to rhythmically fluctuate in both community composition and gene expression in a circadian-dependent manor with respect to host feeding schedules. Disruption of the microbiome circadian rhythm is associated with metabolic disease in mice. Furthermore, host immune cells have been shown to respond to the resident microbiota in a circadian-fashion. These findings provide insight to a microbiome feature that may be important for early diagnosis and therapeutic intervention in the management of metabolic diseases and potentially many others, as well as how frequent flyers cope with changes in sleep patterns. The microbiome circadian rhythm is a largely underexplored field that will likely have profound implications to both the understanding of how bacterial symbiosis affects human physiology and how manipulation of the microbiome may be translated into therapeutic treatment or diagnostic for human disease(s).