Parents, here's another reason for your kids to play outdoors in the dirt: It may make them smarter.
And, as a side benefit, dirt appears to be a natural anti-anxiety drug, but without the side-effects.
Mice exposed to a bacterium found in soil navigated a maze twice as fast, and with less anxiety, as control mice, in studies presented yesterday at the 110th general meeting of the American Society for Microbiology in San Diego.
The researchers say we've become so urbanized we risk losing a connection with an organism in nature that may actually be beneficial to humans.
Dr. Dorothy Matthews became intrigued by mycobacterium vaccae -- a natural soil microbe -- in 2007, when British scientists published a study showing that when mice were injected with a heat-killed version of the organism, it stimulated neurons in the brainstem to start producing serotonin.
"Serotonin is a molecule that has a number of different effects, but one of them is modulating mood and decreasing anxiety," says Matthews, an associate professor of biology at The Sage Colleges in Troy, N.Y.
Serotonin also plays a role in learning. "If you're nervous, if you're frightened, you just can't think straight," Matthews said. She wondered, could M. vaccae have an effect on learning in mice?
She and her colleague, Dr. Susan Jenks, also a professor of biology, dabbed the bacteria onto tiny peanut butter sandwich squares made with Wonder bread.
In one experiment, they tested how long the mice took to navigate a maze, which illustrates how quickly the rodents were learning whether they needed to turn right or left.
The bacteria-exposed mice consistently ran the maze twice as fast as the non-exposed mice. They also showed fewer anxiety behaviours -- less freezing, wall-climbing, stopping and grooming, returning to the start, or defecation.
In other words, they were not scared poop-less.
Next, the researchers removed the bacteria from the peanut butter treats. About one week out, the experimental mice started running the mazes slower than they did when they were ingesting the bacteria. "They experienced a kind of serotonin withdrawal," Matthews said. They were still faster than the controls, on average, an effect that lasted for another month of testing.
After a three-week rest, the bacteria-exposed mice still ran the maze faster than the control mice, but the difference was no longer statistically significant.
Matthews says people are exposed to M. vaccae just by virtue of being outdoors. "If you think about it, when we look at our evolutionary history, we spent a lot of time as hunter-gatherers, or even more recently in agriculture, where we had lots of contact with the soil. It's only been the last 100 years or so that we've become more urbanized."
We no longer eat foods that we grow or gather ourselves, she says -- foods that haven't been "washed multiple times, and dunked in hot water, or processed or grown with pesticides."
Matthews doesn't know how well the bacterium aerosolizes, "but certainly if you're vigorously working in the soil, there are probably some particles that are becoming airborne, so we may very well be inhaling it, as well as eating it by inhaling it and having it get into our GI [gastrointestinal] tract."
We're also exposed via contact with food, especially foods grown directly in the soil, such as carrots and lettuce "and other things that are close to the soil."
Making time in school curriculums for children to learn outdoors may decrease their anxiety and improve their ability to learn new tasks, she says.
"There's a movement now in some schools to actually have gardens that are part of the school experience."
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