Microbiologists at The Sage Colleges in Troy, New York, found that feeding mice a bacteria that is commonly found in soil had a powerful effect on how well they figured out how to get through a complex maze. It also eased their anxiety.
"We were very surprised" at how much improvement the bacteria, Mycobacterium vaccae, made in the performance of mice that got the bacteria compared to mice that didn't, researcher Dorothy Matthews said in a telephone interview.
Matthews and a colleague, Susan Jenks, presented their results during a recent meeting of the American Society for Microbiology in San Diego.
The experimental mice found their way through the maze in half the time it took for a control group that did not receive the bacteria, Matthews said.
And it wasn't a simple task, she added. The mice had to make eight decisions about which pathway to follow, "so it was complicated. If I was going to a place where I needed to make eight decisions about turning left or right, I would consider that a pretty high order task," she said.
So does that mean we all need to start eating mud pies? Not really, she added. The bacteria is so common that we are probably exposed to a sufficient dose every time we walk down a path, or work in the garden, or simply dig around in the dirt.
The research builds on previous work published by others in England in 2007. That research showed that the bacteria injected into mice stimulated growth of some neurons in the brain that resulted in increased levels of serotonin and decreased anxiety.
"The notion that a bacterium that could influence the function of the brain in a positive way was fascinating to me," Matthews said. Serotonin is known to stimulate learning, and the researchers wanted to know how much it helped mice in a controlled lab experiment.
Twenty immature 38-day-old-mice were recruited for the task. They were given a couple of weeks to mature and feel comfortable in their new home. The mice learned that if they found their way to the end of a maze, they were rewarded with slab of bread with a bit of peanut butter.
The mice were divided into two groups of 10, one experimental and one control. The experimental mice received the "peanut butter sandwich" at the end of the maze, and it had a sprinkling of the bacteria. The second group also got the sandwich, but without the bacteria. Each mouse ran the maze 12 times over a three-week period. The difference in performance was quite striking, Matthews said.
The experimental mice found their way through the maze with little trouble, and much less anxiety. The control group, deprived of the bacteria, had a tough time. They would freeze from time to time, and sometimes return to the starting point. They would also try to climb the walls and stop to groom their faces, "like biting your nails when you are anxious," Matthews said.
Meanwhile, she added, the experimental mice stayed "focused" on the task, completing it in half the time it took their bacteria-deprived colleagues.
But alas, the benefit didn't last forever.
Two weeks later the mice ran the maze again, but this time nobody got the bacteria. The experimental mice were still quicker than the controls, but this time they weren't like greased lightning. After three weeks' rest the test was conducted again, without the bacteria. The experimental mice were still a tad faster, but not significantly different.
So the effect was temporary. But why did it work at all?
The bacteria stimulates a group of neurons within the brain to start secreting serotonin in the forebrain area, the part of the brain that's responsible for anxiety and for higher order thinking, and the formation of memories, Matthews said. She suspects its probably part of our evolution.
"For most of our evolutionary history we were hunter-gatherers, rooting around for vegetables, and following animals," she said. "More recently we were agricultural and also having a lot of contact with the soil and with nature," and thus exposed to the bacteria on a regular basis.
What's particularly neat about this work is there isn't any need for some company to bottle the bacteria and sell it to us. The bacteria used in this experiment was live, unlike bacteria that might be found in a supplement, and it's the same stuff we get when we rake the leaves.
"It's healthy for us to have contact with nature," said Matthews, an avid gardener. "Maybe we should design our schools and our living spaces to put us more in contact with nature. It's a healthy thing to do."
So no one needs to eat dirt. Besides, if eating dirt makes us smarter, wouldn't we all be smart enough to eat more dirt?
The better answer, Matthews said, is to take a walk through the woods.