The elusive rhythms of bacterial life.

Circadian clocks are endogenous timekeeping mechanisms that are phylogenetically widespread. Despite the immense diversity of bacterial life, to date, clocks have been identified in few bacterial species. The cyanobacterial clock is understood in great detail, and the roles of its clock proteins in other types of timing mechanisms and in stress resistance are being studied in an ever-growing range of species. Studies of host-associated microbiomes have shown that host and microbial rhythmicity impact one another reciprocally. However, bacterial rhythms have primarily been studied in species in isolation or in host-associated microbiomes. Here, we summarize the state of the field of microbial chronobiology and propose the hypothesis that rhythmicity could be an emergent property of microbial interactions in free-living bacterial communities.