Circadian Organization of the Gut Commensal Bacterium Klebsiella aerogenes

While the expression of circadian rhythmicity is nearly universal among eukaryotic organisms, demonstration of this phenomenon in prokaryotes has been largely restricted to photosynthetic cyanobacteria until very recently. Growing interest in gastrointestinal microbiomes has revealed a complex temporal relationship between the clock of gastrointestinal track and the microbiome within. We have discovered that at least one member of the gut microbiome, Klebsiella (nee Enterobacter) aerogenes, responds to the indoleamine hormone melatonin, secreted by the gastrointestinal system itself, in a specific, dose-dependent fashion such that melatonin increases bacterial motility. Further research revealed that K. aerogenes also express a circadian rhythm in motility and gene expression that is temperature compensated if maintained in constant temperatures ranging from 27 °C to 40 °C. Although rhythmicity is unaltered by changes in constant temperature, cycles of ambient temperature entrain circadian clock in K. aerogenes. Circadian rhythms in these bacteria rapidly decrease in amplitude following exposure to temperature cycles. The mechanisms of this damping are discussed.