By now, the old saw, “You are what you eat,” has been well-used in describing the microbiome. However axiomatic that phrase may be, a new study has also found that who and when that consumption is done can affect microbiome make-up. Changes in the abundance of mouse gut bacteria, over a 24-hour cycle, particularly in females, is tied to rhythms in the internal clock, according to work published online this week in the Early Edition of the Proceedings of the National Academy of Science, by researchers from the Perelman School of Medicine at the University of Pennsylvania.

In mammals, most physiological, metabolic, and behavioral processes follow a daily, or circadian, rhythm, adapting to changing light in the environment. Recent studies of the mouse microbiome have found that the microbes that live in mammals have their own circadian behavior, which has been shown to be linked to host feeding time. However, how this all ties together has not been fully clarified.

Doctoral student Xue Liang, working with the labs of authors Garret A. FitzGerald, MD, FRS, chair of the department of Systems Pharmacology and Translational Therapeutics, and Frederic Bushman, PhD, chair of the department of Microbiology, analyzed circadian rhythms in abundance and type of microbiota in the gut and feces of mice using genetic sequencing. They found that the absolute abundance of Bacteroidetes, a large group of rod-shaped bacteria common in the gut and skin of animals, and relative species make-up of the microbiome, changed over a 24-hour cycle. What’s more, this rhythmicity was more pronounced in female mice.

Normally, during the daytime, when the mice are resting and consuming less food, Bacteroidetes are predominant, reaching the highest abundance toward the end of the light phase. The influence of food may reflect the varying nutrient availability to bacteria. Components in host diet and mucus in the intestines provide carbon sources to the bacteria. Several strains of Bacteroidetes have evolved to use host mucous carbon when dietary carbon is in short supply, which may partially explain the blooming of Bacteroidetes in the resting phase.

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