Unorthodox circadian rhythms in mice lacking commensal microbes

Whether commensal microbes are necessary for generation and expression of mammalian organismal behavioral circadian rhythms has not yet been directly tested. Here, we determine that the gut microbiome is essential for normal host circadian rhythms by comparing locomotor activity (LMA) and core body temperature (Tb) across freely-behaving mice devoid of all commensal microbes (germ-free, GF) and conventionally raised counterparts. GF mice exhibited markedly atypical circadian function, characterized by higher-amplitude LMA and Tb rhythms, and a lengthened circadian period in constant darkness. Circadian entrainment to food was augmented in GF mice, but phase resetting responses to light were unaffected. Conventionalization of GF mice restored normal circadian rhythms, revealing that the concurrent activity of microbes modulates host circadian networks. Whereas the metabolic and obesogenic effects of a Western high-fat, high sugar diet (HFD) did not manifest in GF mice, its circadian-disruptive effects persisted, indicating that HFD impacts behavioral and thermoregulatory circadian rhythms via mechanisms separable from established, microbiome-mediated effects of diet on clock gene networks in the liver. The data indicate that transkingdom interactions are necessary for normal expression of host circadian rhythms: commensal microbes participate in the generation and entrainment of host circadian rhythms in multiple traits and aid in coupling and stabilizing the organismal circadian network.