Lower oral microbiome diversity linked to higher depression symptoms

A rich and varied mix of microbes in the mouth, particularly bacteria, is not only important for our oral health, but may hold clues for other conditions. A new study led by researchers at NYU Rory Meyers College of Nursing finds that a lower diversity of microbes in the mouth is associated with depression.

The mouth is home to between 500 billion and 1 trillion bacteria-the second-largest community of microorganisms in our bodies, after the gut. A growing number of studies point to the connection between the oral microbiome and our overall health, from diabetes to dementia, a relationship thought to be driven by inflammation and disruptions to the immune system.

A recent study in mice found a potential link between depression and the diversity of bacteria in the gut, but less is known about the connection between the oral microbiome and depression.

"Having a better understanding of the relationship between the oral microbiome and depression could not only help us learn about the mechanisms underlying depression, but could contribute to the development of new biomarkers or treatments for mood disorders," said Bei Wu, vice dean for research at NYU Rory Meyers College of Nursing and the senior author of the study, published in the journal BMC Oral Health.

To explore the connection between the oral microbiome and depression, the researchers examined survey and biological data from the National Health and Nutrition Examination Survey (NHANES), conducted by the Centers for Disease Control and Prevention. Using data from more than 15,000 US adults ages 18 and older collected between 2009 and 2012, the researchers compared questionnaires measuring symptoms of depression with saliva samples. Gene sequencing was used to identify the microbes in the saliva and measure the diversity of the oral microbiome.

The researchers found that people with less diversity in their oral microbiomes were more likely to have symptoms of depression. Additional analyses showed that smoking, drinking, and dental care-all of which can change the makeup of bacteria in the mouth-influenced the relationship between the oral microbiome and depression.

These findings suggest that, with more research, the oral microbiome could potentially be used to diagnose or treat depression. However, based on this study, it's not clear whether the diversity of microbes in the mouth influences depression or if depression leads to changes in the oral microbiome-or if there's a bidirectional relationship between the two. 

It's possible that the oral microbiome influences depressive symptoms through inflammation or changes to the immune system. Conversely, depression can drive changes including dietary intake, poor oral hygiene, increased smoking and drinking, or the use of medications-all of which have the potential to alter the oral microbiome. We need more research to understand the direction and underlying pathways of this relationship."

Bei Wu, vice dean for research at NYU Rory Meyers College of Nursing 

"This work is part of a broader effort to understand how the oral microbiome influences not only mental health, but also cognitive decline and the onset of dementia," she added.

Additional study authors include Xichenhui Qiu, Ting Xu, Yiqing Huang, Changning Wei, and Lina Wang.