Chronobiomics: The Biological Clock as a New Principle in Host–Microbial Interactions

The rotation of the Earth around its own axis creates a fundamental challenge for life on this planet, related to the need of all living organisms to modify their physiology in accordance with daily variations in multiple geophysical properties, including light, temperature, and availability of nutrients. As a consequence, all domains of life have developed intrinsic timing systems (“biological clocks”) in order to efficiently adapt organismal activity to the fluctuating environmental conditions [1]. The mechanisms by which different organisms have solved this challenge are manifold. In mammals, the molecular clock is classically understood as a network of transcription factors that are rhythmically operational in virtually all cells of the body [2]. Specifically, the core circadian clock is constituted by the transcription factors Period/Cryptochrome and BMAL1/CLOCK, which not only control each other’s circadian expression pattern but also drive rhythmic oscillations in a large number of target genes, thereby orchestrating the daily activity profile of a cell [3]. Interestingly, mammalian molecular clocks are autonomous and self-sustained but can be entrained by external stimuli, such as light and nutrient availability, in order to adapt organism-intrinsic rhythmicity to fluctuating environmental conditions.