Probiotics may help ease gut disorders linked to long-term stress such as Crohn's disease, research suggests.
Numerous probiotics are available
A team at Canada's McMaster University analysed gut tissue taken from rats put in stressful situations.
Animals fed drinking water containing probiotic bacteria showed less signs that harmful bugs were mobilising to cause damage.
The gut study suggests probiotic bacteria literally crowd out their harmful peers.
Chronic stress is known to be implicated in the development of irritable bowel syndrome and in the worsening of symptoms of inflammatory bowel disease, such as Crohn's and ulcerative colitis.
It also sensitises the gut, producing allergies to certain foodstuffs.
The researchers designed their experiments to try to produce psychological stress in rats similar to that seen in humans. This was done by placing the animals on small platforms surrounded by water.
Half the rats were fed drinking water containing probiotic bacteria in the form of Lactobacillus helveticus and Lactobacillus rhamnosus for a period of seven days before and during the stress sessions.
Analysis showed that exposure to stress made the animals' guts 'leaky', and increased the amount of potentially harmful bacteria sticking to the cells lining the gut wall.
Harmful bacteria were also detected in the mesenteric lymph nodes, which drain fluid coming from the intestine, indicating that bacteria had entered the body and activated the immune system.
However, probiotic treatment minimised the changes in chemical signalling and prevented bacterial 'stickiness' and movement to the mesenteric lymph nodes.
The researchers believe that probiotics probably compete for space with harmful bacteria, helping to dampen down inflammatory responses.
They say their use offers a potentially promising approach to the management of intestinal problems caused by stress.
Professor Alastair Forbes, medical director of the digestive health charity Core and an expert in gastroenterology at University College Hospital, London, said the study presented a "reasonable hypothesis".
He warned research on rats was a long way from finding a similar effect in humans.
But he said previous work had shown that people who drank probiotics did not have a significantly raised total level of bacteria - suggesting there might be some truth in the theory that good and harmful bacteria might compete for space.
However, he said it was possible that probiotics also exerted some sort of biochemical effect on other bacteria.
"As we cannot always remove stress, it would be helpful if we could find new ways to ameliorate its effects," he said.