Inexpensive UV fluorescence approach alerts salmonella presence in two hours

FEBRUARY 9, 2009--Iowa State University (Ames, IA) researchers have developed a technique for determining the presence of salmonella in food within about two hours. The process involves applying a strip of adhesive tape to the suspect food--which in most cases is produce. The tape is carefully removed, taking a sample of whatever is on the skin or surface. That sample is then put on a slide and soaked in a warm, soapy mixture containing a genetic marker that binds with salmonella. The marker emits a fluorescent glow when viewed under an ultraviolet (UV) light.

The fluorescent in-situ hybridization (FISH) approach alerts investigators in about two hours if the produce is contaminated with salmonella. Current methods of detecting salmonella take one to seven days. "This method is rapid, it's easy, and it's cheap," said Byron Brehm-Stecher, assistant professor in food science and human nutrition, who together with graduate student Bledar Bisha, developed the method.

Brehm-Stecher and Bisha call the process "tape-FISH" and note that it could be an important technique for salmonella investigators. "I think this will be good tool in outbreak investigation and routine surveillance especially since all you need is tape, a heat block, a small centrifuge and a fluorescence microscope," said Brehm-Stecher. "It has the potential to be very portable."

Brehm-Stecher's and Bisha's findings will be published in the journal Applied and Environmental Microbiology, published by the American Society of Microbiology.

Once at a location where an outbreak of salmonella has occurred, investigators can test the produce for contamination. Outbreaks can be due to other factors such as food preparation.

Once investigators find the origin of the salmonella, they can take steps to contain it, said Brehm-Stecher.
Salmonella can be found on produce such as tomatoes, cilantro, peppers, spinach and others. The produce can be contaminated while it is in the fields or during processing. Washing the produce thoroughly can help, but cannot ensure the produce will be safe.

The technique could be very valuable as a basic research tool. Researchers could investigate how salmonella and other types of organisms interact on produce surfaces, said Brehm-Stecher. He said he hopes that the approach can help speed investigations of produce contamination, such as last summer's outbreak of Salmonella Saintpaul, which was eventually traced to imported jalapeno and Serrano peppers.

This is the first application of tape-FISH to salmonella, but the idea came to the ISU researcher while reading about art restoration.

In 2008, Brehm-Stecher read about an Italian group that was using a similar approach to look for bacteria on ancient catacombs. Those researchers were hoping to identify and remove bacteria that were slowly eating away at the relics.

After some classroom discussion with his students, Brehm-Stecher decided that using the FISH on produce could be useful and began researching the idea with Bisha. Together, they were able to apply the method to produce and made several improvements in speed and sensitivity over the existing tape-FISH approach.

Posted by Barbara G. Goode, barbarag@pennwell.com.

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