A group of Japanese researchers became the first in the world to discover that bacteria in the salivary microbiome exhibit circadian rhythm and act as if it were a part of the human body, just like organs and cells.

A circadian clock of a 24-hour cycle exists in most organisms including humans, adjusting the circadian rhythm to control various biological functions which occur during the day and at night. The circadian clock transmits information to cells and organs in the body from the brain and sustains the organism's health. Until recently, circadian rhythm was a phenomenon mostly observed in cells and organs of organisms, but the researchers found that it also exists in the microbiome of saliva of humans.

"We were able to prove that the salivary microbiome exhibit circadian rhythm, and our study will help promote healthier lifestyles," says Masahira Hattori, a professor of life science at Waseda University who participated in the research. "Instead of the conventional and invasive approach of drawing blood, doctors will be able to evaluate a person's stress level and health conditions by checking their saliva. They can look for disturbances in the circadian clock, which are associated with increased risk of developing lifestyle diseases, such as obesity, diabetes and cancer. It will also contribute to finding new ways to diagnose these diseases."

This research is published in DNA Research.

In this study, saliva samples were collected from six healthy men and women with 4-hour intervals for 3 days to examine changes in salivary microbiomes longitudinally during the day. Using the metagenomic analysis, the researchers studied in detail the types of bacteria in the salivary microbiomes and the oscillation of its composition.

The analysis revealed the following:

  • 68 to nearly 90% of the microbes detected in the salivary samples significantly oscillated in a 24-hour cycle. Clearer circadian rhythm pattern was observed in bacteria with higher relative abundance.
  • Oscillation patterns differed among those bacteria exhibiting circadian rhythm, some being diurnal while others were nocturnal. For example, Streptococcus significantly increased from dusk to dawn, Prevotella from dawn to noon, and Rothia from noon to night.
  • Circadian rhythm of the salivary microbiome was only observed from bacteria in the oral cavity, not in the samples in the in vitro incubation experiments. This implies that circadian oscillation of the salivary microbiome is linked to a host's physiological changes.
  • An aerobic bacteria oscillates from noon to night, and an anaerobic bacteria oscillates from dawn to noon, meaning that oxygen level in the oral cavity decreases during sleep compared to active hours during the day. There was a tendency for the number of bacterial cells to increase at night, indicating that it also has circadian oscillation.
  • In the evening, gene functions assigned to environmental information processing were enriched, and those of metabolisms such as the biosynthesis of vitamins and fatty acids were enriched in the morning.

These observations suggest that human microbiomes across the body interact with various signals in response to the body's physiological conditions and act as if they are a part of the body like organs and cells.

Because the factors or signals affecting the circadian oscillation of the human salivary microbiome are yet known, the research group intends to conduct further investigation and find out the differences in biological roles of diurnal and nocturnal bacteria in the oral cavity.

Article: Circadian oscillations of microbial and functional composition in the human salivary microbiome, Lena Takayasu, Wataru Suda, Kageyasu Takanashi, Erica Iioka, Rina Kurokawa, Chie Shindo, Yasue Hattori, Naoko Yamashita, Suguru Nishijima, Kenshiro Oshima, Masahira Hattor, DNA Research, doi: 10.1093/dnares/dsx001, published 23 February 2017.