Timing the Microbes: The Circadian Rhythm of the Gut Microbiome.

The mammalian circadian clock system, which includes the master clock and peripheral clocks, times different biological processes in reaction to external cues, such as the light-dark cycle. However, the chronobiology of prokaryotic cells is less well understood, except for in cyanobacteria. The recent blooming of gut microbiome research has revealed a critical role for the trillions of microbes residing in the vertebrate gut in determining both health and disease in the host. The question of whether the gut microbiome exhibits circadian oscillation and how it synchronizes with the host circadian clock has attracted considerable interest. In this review, we discuss the time-of-day-dependent compositional and functional structure within the gut microbial community, how it is regulated by the host, and how it reciprocally influences the host circadian clock.