Print edition: 
WASHINGTON — Candace Smyth, a Maryland lawyer, remembers playing in the dirt.
"I took my Barbies out to play with me," said Smyth, who grew up in Alabama. "I was always outside, always in the dirt, until I got too old.
"I wish my girl could play in the dirt more."
But the family's backyard is small, Smyth said. Five-year-old Kate's day care and after-school activities and visits with friends and family sometimes "keep her indoors too much," she added. "And maybe we parents are too organized, too clean."
Now scientists are beginning to think there could be medical and educational benefits to getting dirty.
Dorothy Matthews and Susan Jenks, biology professors at Sage College in Troy, N.Y., tested mice who ate Mycobacterium vaccae, a harmless bacteria or germ found in dirt almost everywhere.
The professors made little sandwiches of white bread, with a smear of the bacteria, topped with peanut butter.
"Mice love peanut butter," said Matthews. "It was their reward when they ran through our tests."
The professors gave one group of mice M. vaccae (pronounced "vah-kay") sandwiches and another group just little peanut butter sandwiches, then watched how quickly the animals could work their way through a difficult maze to the peanut butter reward at the end.
The mice fed M. vaccae navigated the maze twice as fast "and showed far less anxiety" than the other mice, Matthews said.
In a second test, the researchers removed the dirt germ from the first group's diet, but they maintained their learning edge. Three weeks later — which for mice is about the same as 2 1/2 human years — tests showed that mice exposed once to M. vaccae could remember what they learned for a long time.
"That's pretty cool," said Matthews, who remembers growing up in New York City, playing in the dirt of a backyard "with one little tree."
Other scientists who injected M. vaccae into mice found it stimulated neurons in the brainstem to start producing serotonin, she said. Humans make serotonin in their bodies naturally, and it is a well-known contributor to feelings of well- being.
So does that mean dirt sandwiches are a new vitamin for learning?
"Oh, no," explained Matthews. "Please do not start eating dirt sandwiches." It will take years of studies to find out if there is a real benefit from M. vaccae on children or adults, she said.
"But Mother Nature knows best," she added. "It's good for us to be outside on a lot of levels. People feel better when outside and active, and even on a chemical level, that exposure to the biologic world that in all likelihood we evolved with could help us live better lives."
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