This study from 2007 (press release here) showed that mice fed the bacteria Mycobacterium vaccae found in soil, had similar effects as antidepressant drugs. The study started after cancer patients given M. vaccae reported improvements in their quality of life, leaving scientists wondering if these bacteria might alter serotonin levels. The lead author of the study said:
“These studies help us understand how the body communicates with the brain and why a healthy immune system is important for maintaining mental health. They also leave us wondering if we shouldn’t all be spending more time playing in the dirt.”It makes sense that our bodies are meant to ingest some dirt. It may be that we clean our food a little too well. It's a widely held belief that vegans can’t get enough vitamin B12, but if we simply left some dirt on our fruits and vegetables we'd get plenty of B12 from the soil.