While always delightful, Jan's mood perks up even more in spring when she can work in the garden and especially compost. It may not be just the pastime - it could be a chemical reaction to dirt.
A lack of serotonin is linked with depression in people.
The scientists say more work is now needed to determine if the bacterium Mycobacterium vaccae has antidepressant properties through activation of serotonin neurons.
Lead researcher Dr Chris Lowry 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 spend more time playing in the dirt." [More]
We already suspect living dirty may boost our immune system as well.
Of course, this development might have a few implications for others - like say, farmers.