Life on Earth is intimately connected to the natural cycles of light and dark that make
up a 24-hour day. For plants, animals, and even bacteria, these circadian rhythms
control many biological functions. Humans can overrule their body clocks, but at a
price: People whose circadian rhythms are regularly disrupted—by frequent jet lag or
shift work, for example—are more vulnerable to diabetes, obesity, cardiovascular
disease, and cancer. There are various theories to explain these associations, and
researchers now have a new player to consider: the bacteria that live in the digestive
tract. According to a study in mice and a small group of human volunteers, the internal
clocks of these gut microbes sync up with the clocks of their hosts. When our circadian
rhythms get out of whack, so do those of our bacteria.
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.