Maybe its not that simple, but exposure to probiotic bacteria commonly found in soil have been linked to the brain chemical, serotonin.

• Cancer patients increased their qualities of life with exposure to Mycobacterium vaccae
• M. vaccae activated a group of neurons to produce more serotonin mitigating depression

Full article at Getting Dirty May Lift Your Mood.

probiotic, bacteria, brain, chemical, serotonin, cancer, neurons, depression