Interplay between Microbes and the Circadian Clock.

Circadian rhythms influence virtually all life forms on our planet, a notion that opens the question on how the circadian cycles of individual organisms may interplay with each other. In mammals, a potentially dangerous environmental stress is represented by encounters with infectious agents. Microbial attack is a major risk for organismal homeostasis and therefore needs to be efficiently counteracted by mechanisms implemented by the host immune system. Accumulating evidence shows that the immune system may anticipate an emerging pathogenic exposure through an enhanced inflammatory state. Notably, the circadian clock orchestrates these anticipatory responses to fluctuating conditions in the external world. In this article, we review the current knowledge about the relationship between the circadian clock and pathogenic infections. We discuss the role of the circadian clock against infection and specific pathogens, the core clock proteins involved in the defense mechanisms, and the specific tissue or cell type in which they function to counteract the infection. Finally, circadian oscillations in the gut microbiome composition and its possible role in protecting against foodborne pathogen colonization are presented.