Life on Earth is closely linked to natural cycles of light and darkness that make up a 24-hour day. For plants, animals and even bacteria, these circadian rhythms control many biological functions. Humans may ignore their body clocks, but at a price: the people whose circadian rhythms are regularly disrupted by jet lag or frequent shift, for example, are more vulnerable to diabetes, obesity, cardiovascular disease and cancer. There are several 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 synchronize with the clocks of their hosts. When our circadian rhythms are out of whack, and those of our bacteria.
Recent years have seen an explosion of interest in the constellation of bacteria that call home to the gut, and these microbes appear to play a role in any of immunity metabolism mood. But although the broken bacteria are observed in many of the same diseases that arise from asymmetric circadian rhythms, the exact link is not fully understood. Eran Elinav a specialist and immunologist microbiome at the Weizmann Institute of Science in Rehovot, Israel, wondered if microbes own circadian rhythms were a missing piece of the puzzle.
To test the theory, he and his colleagues analyzed the bacteria in fecal samples of laboratory mice maintained normal 12-hour cycles of light and darkness. Samples were taken every 6 hours for two cycles of 24 hours. Up to 60% of the microbes is composed of different types of bacteria that fluctuated in both the number and frequency with respect to each other, throughout the day and night. During the dark phase (when mice are nocturnal, are most active) bacteria are digesting nutrients busy repairing their DNA, and growth, as shown by the different activity of the bacterial gene documented from fecal samples collected at different time points. During the light phase, microbes went on ongoing process "housekeeping" such as detoxification, detection of chemicals around them, and the construction of the flagella, or tails, which help microbes move.
In mice with a mutation that disables the internal clock, gut bacteria do not show the same changes in both population and activity in response to light and darkness , suggesting that the animal of clock control in some way that of bacteria. When bacteria from these mice "no clock" were transplanted into healthy animals living in normal light-dark conditions, microbes began showing normal rhythms in a week.
The results, reported online yesterday in Cell came as a surprise, said Elinav. Previous studies have shown that many bacteria have circadian clocks cyanobacteria sensitive to light, for example, which derive their energy from photosynthesis. But the deep microbes in well guts, entrails-spend all their time in the dark. How did they know what time of day it was? Some signal must pass the host bacteria.
A major difference between normal mice and those of the clock was disabled when the animals ate, the researchers found. Normal mice eat at night when they are active; mice without clock ate almost continuously. So could mealtime is the signal? When the researchers changed the eating habits of animals by normal feeding mice during the light cycle (the night of a mouse), numbers, types, and activity of bacteria and displaced. The researchers also found that mice whose light-dark cycles were disrupted gained weight and developed physiological changes associated with diabetes, such as insulin resistance. Because humans with irregular sleep patterns also tend to eat more at night, researchers suspect that these eating habits contribute to disease specifically disrupting gut microbes.
The bacteria are probably not the whole story; irregular sleep and eating may contribute to the disease through other channels, such as excess stress hormone and insulin production. Although, "This is a compelling study," says microbiologist Rob Knight of the University of Colorado, Boulder. Knight said some of the strongest evidence for a role of bacteria in the circadian related diseases is the final phase of the study, when the research team analyzed fecal samples from two people on a normal schedule and two others who had recently stolen from the United States to Israel. Analysis of the samples before, during and after the fighting jet lag, they found similar bacteria fluctuations in what they saw in mice. jetlag of participants showed an increase in a type of bacteria known to be more common in people with diabetes and obesity; the levels of these microbes have dropped to normal once the travelers adjusted to the new time zone.
The most compelling of all, Knight believes, is that when samples of gut bacteria of human jetlag were transplanted into healthy mice, the animals gained weight showed an increase blood sugar, body fat and had a higher content compared to animals receiving the bacteria participants before their flight.
so we can ward off evil effects of jet lag by being more careful about how or when we eat? At this point, "he is an educated guess," said Elinav.
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