Mycobacterium vaccae – The Mood-Boosting Microbe

When we think of microbes, we often imagine germs that cause illness or bacteria that help us digest food. But some microbes may also influence how we feel. Mycobacterium vaccae, a harmless bacterium found in soil and water, has captured scientific interest for its potential effects on both body and mind. Sometimes called the “mood-boosting microbe,” it has become a symbol of how invisible organisms can shape human wellbeing in surprising ways.

A relative with a difference

Mycobacterium vaccae belongs to the same genus as Mycobacterium tuberculosis, the cause of tuberculosis. The family connection is enough to raise eyebrows – but unlike its notorious cousin, M. vaccae is nonpathogenic and safe. Instead of invading lungs, it lives quietly in soil, mud, and water, helping to break down organic material. Its presence reminds us that many bacterial relatives differ dramatically in their effects on humans – some dangerous, others benign, and a few possibly beneficial.

Nature’s antidepressant?

What has made M. vaccae particularly intriguing is its possible role in mental health. In animal studies, mice exposed to the bacterium showed reduced anxiety and improved performance in stress-related tests. Research suggests that M. vaccae may stimulate the release of serotonin in the brain – the same neurotransmitter targeted by many antidepressant drugs.

While these findings remain experimental, they provide a biological explanation for something people have long intuited: that spending time outdoors, gardening, or simply playing in the dirt has positive effects on mood. Some researchers now wonder if contact with soil microbes like M. vaccae could partly explain the mental health benefits of nature. Though not yet a medical treatment, the idea that a microbe could support emotional resilience continues to inspire investigation.

Training the immune system

The influence of M. vaccae is not limited to mood. Research also indicates that it has immune-modulating properties. It appears to promote anti-inflammatory responses, balancing immune activity in ways that may reduce allergic reactions and support long-term health.

Because of these effects, M. vaccae has been studied as both a vaccine for prevention of tuberculosis, and as a therapy for people who already have tuberculosis as well as in therapies for asthma and allergy control. Immunologists place it among the “old friends” – microbes common in soil, water, and animals, with which humans coexisted throughout evolutionary history. Regular exposure to such microbes may have trained our immune systems, preventing overreactions that today manifest as allergies, autoimmune conditions, or chronic inflammation. In this sense, M. vaccae serves as a reminder that our relationship with microbes is not just about fighting pathogens but also about maintaining balance.

From soil to science

Ongoing research is exploring whether M. vaccae could be harnessed for therapies that extend beyond the immune system. Studies are testing its potential to reduce symptoms of depression, help individuals cope with post-traumatic stress disorder (PTSD), and even build resilience to stress in general. These investigations remain preliminary, and medical applications are not yet established. Still, the work reflects a broader shift in science – recognizing that soil microbes may play a role in mental and emotional health, not just physical survival.

A microbe with a gentle message

Whether or not it becomes part of future treatments, Mycobacterium vaccae carries a broader message. It reminds us that health is not simply a matter of avoiding dangerous microbes, but also of embracing the beneficial ones. Just as probiotics highlight the role of gut bacteria in digestion, M. vaccae suggests that our connection to the microbial world extends to our minds.

When we dig in the soil, plant a garden, or walk barefoot on the ground, we may be doing more than connecting with nature – we may be renewing an ancient partnership with microbes that once shaped our evolution and continue to influence us today.

From dirt to wellbeing

The story of M. vaccae illustrates how deeply entangled human health is with the microbial environment. From its ability to calm anxious mice to its potential to train our immune system, this soil bacterium bridges the gap between modern science and ancient wisdom. It embodies the idea that nurturing the earth may also nurture ourselves – a reminder that our wellbeing depends not only on medicine and technology but also on the invisible life all around us.

Text developed in collaboration with Prof Christopher A. Lowry, University of Colorado Boulder