What can our spit tell us about our mental health? Quite a bit actually. Researchers have recently found that the microbes found in the saliva of pregnant people could reveal whether they’re currently experiencing stress, anxiety, depression, and post-traumatic stress disorder symptoms.

This is significant because while good mental health supports maternal well-being and healthy infant development, about 20% of pregnant women globally experience poor mental health.

While the gut microbiome is the most studied community of microbes on our bodies, there’s less known about the microbes that live in our mouths and how they impact our health.

“There’s a really burgeoning literature on the lower gut microbiome and mental health in humans, but there’s only a small handful of studies that have looked at associations between the oral microbiome and mental health,” said Joe Lonstein, a professor in behavioral neuroscience at Michigan State University who led this study. “None were in a pregnant population.”

By examining the connections between the oral microbiome and mental health in pregnant people, this study could not only help identify those in need of help, but could also provide a starting point for possible points of intervention to improve outcomes for pregnant people and their infants.

This study included 224 pregnant people in their second trimester. Participants collected saliva using at-home kits and in the same week, they completed surveys on the stressors they were experiencing and their mental health symptoms.

Lonstein and his colleagues identified the microbes present in the saliva based on their DNA sequences and looked for correlations between the microbes identified and mental health symptoms from the surveys.

They found that not all mental health symptoms correlated with the same set of microbes in the mouth. Lonstein explained, “if you had high [situational] anxiety, that was related to one set of microbes. If you have PTSD symptoms, that was related to a different set.”

In addition, this study found that the microbes from the saliva samples were different from what previous research has found about the connections between gut microbiome and maternal mental health. “The oral microbiome is not just a mirror of the gut microbiome,” Lonstein said.

Correlation or causation?

One of the biggest challenges across the microbiome field is understanding whether the correlations seen between the microbiome and health indicate true cause and effect. .

For instance, do changes in the oral microbiome actually cause changes in our mental health? Or do changes in our mental health cause changes to the microbiome? Or, perhaps the effects go both ways.

These questions are hard to answer in a study like the one conducted by Lonstein, as it only measures at one instance in time. To really understand the relationships between the oral microbiome and mental health, Lonstein said “what you would need is a longitudinal study where you measure stress and mental health symptoms multiple times across pregnancy.”

An example of such a study examined how the gut microbiome affects anxiety and depression symptoms during pregnancy and postpartum. This study found that administering a probiotic beginning in the second trimester reduced anxiety and depression symptoms at six months postpartum.

However, it’s still unclear how the oral microbiome and mental health are connected. Lonstein added, “there’s exciting potential for future work to show that there are causal relationships with mental health.” Some mechanisms that scientists are currently exploring include specific bacteria in our mouths changing the levels of pro-inflammatory molecules that affect the brain, altering neurotransmitter synthesis, or making their way to the gut to influence mental health.

Can probiotics or oral hygiene help?

If it’s possible that the state of the oral microbiome can affect our mental health, could taking an oral probiotic help promote mental wellbeing in pregnant people? The answer is complicated and depends whether or not the oral microbiome causes changes in mental health or only reflects mental states.

If the oral microbiome does cause changes to mental health, Lonstein says oral probiotics could help mental well-being, but would have to act directly in the mouth. Think: toothpaste. “If you just swallow them, that may not have that much of an effect [directly on the oral microbiome],” he explained. Scientists have also seen differences in the oral microbiome linked to diet.

However, changes to the oral microbiome and the diet can also influence the microbes in the gut microbiome so it’s hard to tease apart whether the oral microbiome directly affects mental health or whether changes to the oral microbiome alters the gut microbiome which then affects mental health.

“Those things are very intertwined,” said Lonstein. “It probably will be very difficult to ever know very clearly whether you can change just the mouth, or just the gut without changing the rest of the mouth.”

References: Joseph S. Lonstein, et al., Stress and mental health symptoms in early pregnancy are associated with the oral microbiome, BMJ Mental Health (2024). DOI: 10.1136/bmjment-2024-301100; E.A. Mitchell, et al., Effect of Lactobacillus rhamnosus HN001 in Pregnancy on Postpartum Symptoms of Depression and Anxiety: A Randomised Double-blind Placebo-controlled Trial, eBioMedicine (2017). DOI: 10.1016/j.ebiom.2017.09.013