Scientists found that a certain class of bacteria found in the human gut, Enterobacter aerogenes, expresses circadian patterns because of its sensitivity to melatonin, the hormone produced at night and stimulating sleep. While melatonin is made by the pineal gland, a small gland in the brain, it is also present throughout the gastrointestinal (GI) system. In addition, many foods contain melatonin. The GI system's circadian clock is coordinated to both light and the timing of eating. Scientists said the effect of melatonin on this bacterium is remarkable: when exposed to melatonin at levels similar to those found in the gut, the individual cells begin to communicate with each other and coordinate periods of swimming and dividing in a phenomenon known as swarming. This is the first demonstration of a circadian clock in a prokaryote outside the phylum Cyanobacteria, and the researchers' findings suggest that the cyanobacterial and E. aerogenes clocks share common evolutionary ancestors. http://medicalxpress.com/news/2016-08-gut-bacteria-circadian-clock.html Study: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0146643
