"That's good news for people who do not like to consume a lot of sugary juice," Terri Camesano of Worcester Polytechnic Institute in Massachusetts said in a statement.

Camesano said unpublished work shows that while cranberry juice has potent effects on disease-causing bacteria, those effects are transitory.

"When we take E. coli. bacteria that have been treated with cranberry juice and place them in normal growth media, they regain the ability to adhere to urinary tract cells," Camesano said in a statement. "This suggests that to realize the anti-bacterial benefits of cranberry, one must consume cranberry juice regularly -- perhaps daily."

The juice prevents the microorganisms from getting close enough to latch onto cells and initiate an infection, Camesano said.

The study, published in the journal Colloids and Surfaces: B, shows 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.