Are your bacteria jet-lagged?

Are your bacteria jet-lagged?

Lacelike bacteria in the bumpy tissue that lines the mouse gut respond to disruptions in their host’s circadian rhythms.

(http://news.sciencemag.org/biology/2014/10/are-your-bacteria-jet-lagged)

Transkingdom Control of Microbiota Diurnal Oscillations Promotes Metabolic Homeostasis

(http://www.cell.com/cell/abstract/S0092-8674(14)01236-7)

The last several years have seen an explosion of interest in the constellation of bacteria that call the gut home, and these microbes appear to play a role in everything from immunity to metabolism to mood. But although disrupted bacteria are observed in many of the same diseases that arise from skewed circadian rhythms, the precise link isn’t fully understood. Eran Elinav, an immunologist and microbiome specialist at the Weizmann Institute of Science in Rehovot, Israel, wondered whether the microbes’ own circadian rhythms were a missing piece of the puzzle.

To test the theory, he and his colleagues analyzed bacteria in fecal samples from lab mice kept in normal 12-hour cycles of light and darkness. Samples were taken every 6 hours for two 24-hour cycles. Up to 60% of the microbes consisted of various bacterial types that fluctuated, in both their total number and their prevalence relative to each other, throughout the day and night. During the dark phase (when mice, being nocturnal, are most active), the bacteria were busy digesting nutrients, repairing their DNA, and growing, as evidenced by the various bacterial gene activity documented from fecal samples taken at different time points. During the light phase, microbes went about ongoing "housekeeping" processes, such as detoxifying, sensing the chemicals around them, and building the flagella, or tails, that help the microbes move.

In mice with a mutation that disables the inner clock, the gut bacteria didn’t exhibit the same fluctuations, in either population or activity, in response to light and dark—suggesting that the animal's clock somehow controls that of the bacteria. When bacteria from these "clockless" mice were transplanted into healthy animals living in normal light-dark conditions, the microbes began to show normal rhythms within a week.

The findings, reported online yesterday in Cell, came as a surprise, Elinav says. Previous studies have shown that many bacteria do have light-responsive circadian clocks—cyanobacteria, for example, which get their energy from photosynthesis. But microbes deep in the bowels of—well, the bowels—spend all their time in the dark. How did they know what time of day it was? Some signal must pass from the host to the bacteria.