A new study by researchers at Worcester Polytechnic Institute, published in the journal Colloids and Surfaces, shows that cranberry juice changes the physical properties of the bacteria that cause urinary tract infections, creating a barrier that stops them getting close to the cells of the urinary tract and so helps prevent infection.
There are two kinds of E. coli: one without fimbriae (fine hair-like threads) and one with them. The kind with fimbriae can stick to the cells of the urinary tract. Even at low concentrations, cranberry juice changed measures of how easily the bacteria bind to cells. One of these, the Gibbs free energy of adhesion, measures the energy that must be spent before the bacteria can attach. Without cranberry juice it is negative, meaning attachment happens easily; with cranberry juice it becomes positive, and rises steadily as the juice concentration increases, so attachment becomes unlikely.
When the researchers mixed urinary tract cells and bacteria in solution, the bacteria attached to the cells without cranberry juice, but adding the juice reduced attachment in proportion to the amount added. The juice had no effect on bacteria without fimbriae, so its components act directly on the fimbriae, which became compressed, as seen under the microscope.
"Our results show that cranberry juice affects the bacteria that cause urinary infections, but not harmless organisms, so it won't harm the gut's normal bacteria," says one of the researchers. The effect is temporary: when bacteria exposed to the juice were put back in normal culture, they regained their ability to stick. So the juice works against the bacteria without killing them.
Cranberry products may help reduce repeat urinary infections in some women, but the evidence is modest, and cranberry doesn't treat an infection that has already started. Burning on passing urine, fever or back pain need a doctor. People taking warfarin should check with their doctor before drinking a lot of cranberry juice.
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