Country bumpkins may not deserve their simpleton reputation because living close to the soil could make them brighter than sophisticated urbanites, research suggests.

Scientists in the US have discovered that a common soil bug improves the learning ability of mice. They speculate whether it might also help make humans smarter, especially children.

Previous research indicated that the bacteria stimulate the growth of neurons and alter brain chemistry. The new findings suggest the bug can improve both mood and memory.

Study leader Dorothy Matthews, from The Sage Colleges in Troy, New York, said: "Mycobacterium vaccae is a natural soil bacterium which people likely ingest or breathe in when they spend time in nature.

"This research suggests that M. vaccae may play a role in anxiety and learning in mammals. It is interesting to speculate that creating learning environments in schools that include time in the outdoors where M. vaccae is present may decrease anxiety and improve the ability to learn new tasks."

Previously, dead bacteria injected into mice were found to stimulate the growth of certain brain cells, raising levels of the "feel good" neurotransmitter serotonin.

The brain chemical helps messages pass between neurons. In humans, lowered serotonin activity is linked to depression. Boosting levels of the chemical in mice with M. vaccae was found to reduce anxiety.

In the new research, presented at the American Society for Microbiology's annual meeting in San Diego, California, Dr Matthews' team investigated whether the soil bug also had an effect on learning.

As well as influencing mood, serotonin is known to play a role in learning ability.

Live bacteria were fed to mice that were then timed as they taught themselves how to navigate a maze. Their performance was compared with that of a group of "control" mice not fed the bacteria.

"We found that mice that were fed live M. vaccae navigated the maze twice as fast and with less demonstrated anxiety behaviours than control mice," said Dr Matthews.

The effect lingered for a short time after the bacteria were removed from the animals' diets but then faded away.

When mice were retested having returned to a normal diet they ran the maze more slowly than before, but still faster than animals never given the bacteria.

After a gap of three weeks mice previously fed the bugs were still quicker, but the results were no longer statistically significant.

The pattern suggested that the effects of the bacteria are temporary, said the scientists.