Circadian rhythms are fundamental properties of most eukaryotes, but evidence of biological clocks that drive these rhythms in prokaryotes has been restricted to Cyanobacteria. In vertebrates, the gastrointestinal system expresses circadian patterns of gene expression, motility and secretion in vivo and in vitro, and recent studies suggest that the enteric microbiome is regulated by the host’s circadian clock. However, it is not clear how the host’s clock regulates the microbiome. Here, we demonstrate at least one species of commensal bacterium from the human gastrointestinal system, Enterobacter aerogenes, is sensitive to the neurohormone melatonin, which is secreted into the gastrointestinal lumen, and expresses circadian patterns of swarming and motility. Melatonin specifically increases the magnitude of swarming in cultures of E. aerogenes, but not in Escherichia coli or Klebsiella pneumoniae. The swarming appears to occur daily, and transformation of E. aerogenes with a flagellar motor-protein driven lux plasmid confirms a temperature-compensated circadian rhythm of luciferase activity, which is synchronized in the presence of melatonin. Altogether, these data demonstrate a circadian clock in a non-cyanobacterial prokaryote and suggest the human circadian system may regulate its microbiome through the entrainment of bacterial clocks.

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Published in PLOS ONE, v. 11, no. 1, e0146643, p. 1-13.

© 2016 Paulose et al.

This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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The Cassone (VMC) laboratory is supported by National Institutes of Health R01 AG045833-01, “Circadian rhythms and melatonin signaling in the aging gastrointestinal system”; United States Department of Agriculture NIFA 2014-67012-21608, “The role of the premammillary nucleus in the regulation of broodiness in the domestic turkey” (Mentor; Dr. Ashli Moore post-doctoral fellow); and Howard Hughes Medical Institute Science Education Grant, “Student Retention and Success in STEM through collaborative and multi-layered STEMCats Freshmen Program.”

journal.pone.0146643.s001.TIF (1374 kB)
S1 Fig. Multiple sequence alignment of protein BLAST hits to human MEL1A and MEL1B receptors show several areas of identity with protein sequences taken from the Human Microbiome Project (HMP).

journal.pone.0146643.s002.TIF (3999 kB)
S2 Fig. Exposure to physiological levels of melatonin induce swarming in E. aerogenes.

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S3 Fig. MotA::luxcdabe is expressed rhythmically in E. aerogenes, not in DH5-α.

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S4 Fig. Phylogenetic relationships exist between Cyanobacteria clock proteins and E. aerogenes, not vertebrate clock proteins.

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S5 Fig. Kai protein orthologs in E. aerogenes do not share motif-level similarity with other Kai proteins.

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