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Cranberry juice prevents

nonspecific adhesion

Cranberry juice is commonly used totreat urinary tract infections. Althoughno one knows for sure why it works, sci-entists believe that something in theNorth American cranberry (Vaccinummacrocarpon) prevents the adhesion ofE. coli bacteria to human cells. In thisissue of Analytical Chemistry (pp853–857), Frances Ligler and col-leagues at the U.S. Naval ResearchLaboratory (NRL) show that cran-berry juice also prevents non-specific adhesion of bacteria toborosilicate glass microscope slidesused in an NRL immunoarraybiosensor.

In most cases, nonspecific bind-ing of analytes in the NRL arraysensor is not a problem, saysLigler. “We’ve worked very hardto minimize it,” she adds. Butvery high concentrations of somebacteria do stick to the glass slideused in the sensor.

The NRL array biosensor cansimultaneously detect multipleanalytes in complex samples. Thesystem uses a standard sandwichimmunoassay, in which captureantibodies are immobilized on the sur-face of a glass microscope slide. Samplesflow across the slide, followed by a trac-er solution containing fluorescently la-beled antibodies against the antigens ofinterest. The fluorescence signal identi-fies the location of the antibody-boundantigen.

The sensor has been used to analyzeeverything from food and environmen-tal samples to clinical samples. Most ofthe time, the sensor performs well, butoccasionally analytes are problematic be-cause they stick to the glass slide, saysLigler. High background signals can re-sult from this nonspecific binding. Inaddition, if analytes bind to areas whereantibodies against other analytes are,more false positives will result.

“We’ve probably tried an armament of50 different things” to prevent nonspe-

cific binding, says Ligler. “We came tothe same conclusion as everybody elsethat BSA [bovine serum albumin] anddetergent work as good as just aboutanything,” she adds. But then her post-doc, Brandy Johnson-White, came upwith the idea of using cranberry juice.“It really surprised me because I hadalways thought that cranberry juiceworked because of pH. But after going

to the literature, we found that this is notthe case,” says Ligler.

Some reports suggest that the highsugar content in cranberry juice is respon-sible for preventing biofilm formation.But that is not all there is to it either, saysLigler. “As we started tracking it down,there started to be hints in the literaturethat the tannins were responsible forsome of the in vivo activity,” she says.

Although the researchers haven’tcompletely isolated the tannins, theydo believe that tannins, also known asproanthocyanidins, are responsible forthe anti-adhesion properties of cranber-ry juice. They ruled out all the low-mo-lecular-weight compounds, includingpolyphenols. And they know it’s not thesugars, because sugars were dialyzedaway in their experiments. “We tried toput back in high concentrations of the

sugars, and that didn’t work either,”says Ligler.

The researchers showed that cranber-ry juice prevents nonspecific binding ofseveral food-borne pathogens, includingE. coli, Salmonella typhimurium, andStaphylococcus aureus. Only red cranber-ry juice was successful at preventing thebacteria from sticking to the microscopeslide. As the concentration of cranberry

juice increased from 0 to 50%,the background-to-fluorescencesignal (B/F) ratios decreased.The researchers also tried whitecranberry, grape, orange, andapple juices, but none of themaffected the B/F ratio.

The effect of cranberry juiceon Campylobacter jejuni and Lis-teria monocytogenes was not as ob-vious. In those cases, the B/Fratio remained constant regardlessof the concentration of cranberryjuice. The researchers were notsurprised by this result becausethe antibodies that they used hada high affinity for those antigens.Low B/F ratios can be achievedby reducing nonspecific adhesion,by using antibodies with high af-finity for the antigens, or by com-bining both methods.

Why cranberry juice works is still un-clear. “In fact, we don’t know yet if it isworking on the cells or on the substrate,”says Ligler. The next step is to isolatethe tannins and see what happens whenthey bind to the surface, she says. Iso-lating the tannins won’t be easy, howev-er, because they are very heterogeneous,she adds. The researchers are also inves-tigating whether cranberry juice canprevent proteins from sticking to micro-fluidic channels.

Although several questions still re-main unanswered, the work is interest-ing in that it doesn’t follow the normalway that science progresses. “So manythings go from in vitro observation toin vivo applications. Here, we are goingfrom an in vivo observation to an invitro application,” says Ligler. a

—Britt Erickson

Researchers have shown that cranberry juice preventsnonspecific binding of some bacterial cells in an arraybiosensor.

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