THURSDAY, July 31 (HealthDay News) -- Researchers at Worcester Polytechnic Institute say that have solved the mystery of how cranberry juice prevents urinary tract infections.

Their study, published in the current issue of Colloids and Surfaces: B, shows that the juice changes the thermodynamic properties of bacteria in the urinary tract to form an energy barrier that prevents infections from developing.

The research team found that when bacteria with hair-like projections known as fimbriae, which is present on virulent bacteria that cause urinary tract infections, is exposed to even low concentrations of cranberry juice that energy levels increased to levels that made it difficult for the bacteria attach to a cell.

When the bacteria and urinary tract cells were placed in a solution, fewer and fewer attachments were observed as the concentrations of cranberry juice were added to the mix.

Cranberry juice did not appear to affect bacteria without fimbriae, which suggests something in the juice may directly change the molecular structure of the fimbriae themselves.

"Our results show that, at least for urinary tract infections, cranberry juice targets the right bacteria -- those that cause disease -- but has no effect on non-pathogenic organisms, suggesting that cranberry juice will not disrupt bacteria that are part of the normal flora in the gut," Terri Camesano, an associate professor of chemical engineering at WPI, said in a university news release. "We have also shown that this effect occurs at concentrations of cranberry juice that are comparable to levels we would expect to find in the urinary tract."

Camesano said that unpublished work also shows cranberry juice has potent effects on disease-causing bacteria, but that the effect is temporary. "This suggests that to realize the antibacterial benefits of cranberry, one must consume cranberry juice regularly, perhaps daily," she said.

Other work her lab did shows that regular cranberry juice cocktail and sugar-free cranberry juice both work effectively to prevent urinary tract infections.

More information

The U.S. National Library of Medicine has more about urinary tract infections.