Depression may be linked to a lack of diversity in the bacteria in your mouth, according to a new study. Scientists from New York University say your mouth contains between 500 billion and 1 trillion bacteria—the second-largest community of microorganisms in the human body. The makeup of those microbes diagnoses and treats depression.

In a new study, scientists found that less diversity of microbes in the mouth is associated with mental health conditions, which negatively affect how you may feel, think, and behave. It causes persistent feelings of sadness and loss of interest in activities and impacts various aspects of your daily life. Depression can affect anyone regardless of age, gender, or background.

“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,” Dr Bei Wu, vice dean for research at NYU Rory Meyers College of Nursing, said in a statement. “We need more research to understand the direction and underlying pathways of this relationship.”

For the study, scientists examined data from the Centres for Disease Control and Prevention’s National Health and Nutrition Examination Survey – where questionnaire data from more than 15,000 adults was collected between 2009 and 2012 to compare symptoms of depression with saliva samples.

They also used gene sequencing to identify the microbes in the saliva and measure the diversity of the oral microbiome. That’s how they found that people with less microbial diversity were more likely to have symptoms of depression. Additional analysis revealed that practices including smoking, drinking, and dental care influence the relationship between oral microbiome and depression, which changes the makeup of bacteria in the mouth.

According to experts, antidepressants and other psychotropic medications can have severe side effects that cause saliva to reduce and alter the oral microenvironment. “And so, medications not only alleviate depressive symptoms but also have a regulatory effect on oral health,” they said.

However, it remains unclear if the diversity of microbes in your mouth can influence depression – and if it can lead to changes in the oral microbiome.

They noted that a recent study found a potential link between depression and the diversity of bacteria in the gut and that a growing body of research points to a connection between the oral microbiome and overall health. That’s thought to be driven by inflammation and disruptions to the immune system.

“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 Wu.

According to experts, an estimated 21 million adults had at least one major depressive episode in 2021 – causing a persistent feeling of sadness and loss of interest in things and activities they once enjoyed. It also causes difficulty with thinking, memory, eating, and sleeping. While it may be normal to feel sad about or grieve over difficult life situations, depression, experts believe, is different in that it persists practically every day for at least two weeks and involves other symptoms than sadness alone.

There are several types of depressive disorders. Clinical depression, or major depressive disorder, is often just known as depression. It is the most severe type of depression.