UV Light Detects Salmonella

After the recent outbreak of Salmonella in the US, which has sickened more than 500 people and killed eight, the federal government has launched a criminal investigation that will be carried out by the FDA’s criminal division and the Department of Justice.

Questions and growing concern over quality control have now put sensing technology in the national spotlight.

Iowa State University researchers have developed a technique for testing for the presence of Salmonella that may give investigators faster, more precise answers.

The process, developed by Byron Brehm-Stecher, assistant professor in food science and human nutrition, and his graduate student Bledar Bisha, begins with testing the food – in most cases produce – with a strip of adhesive tape.

Salmonella-contaminated produce – contained in a biosafety cabinet – are tested using Brehm-Stecher and Bisha's tape-FISH method. Image courtesy of Iowa State University.

The tape is applied to the produce, then carefully removed, taking a sample of whatever is on the skin of the produce. That sample is then put on a slide and soaked in a special warm, soapy mixture that contains a genetic marker that binds with Salmonella and that gives off a fluorescent glow when viewed under an ultraviolet light. Use of this genetic marker approach is called fluorescence in situ hybridization, or FISH.

In about two hours, the technique can reveal whether the produce is contaminated with Salmonella. Current methods typically take between one and seven days.

“This method is rapid, it’s easy, and it’s cheap,” Brehm-Stecher said.

He and Bisha call the process “tape-FISH” and note that it could be an important technique for Salmonella investigators.

“I think this will be a 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,” Brehm-Stecher said. “It has the potential to be very portable.”

The investigators’ 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 the result of other factors, such as food preparation.

Once investigators find the origin of the Salmonella, they can take steps to contain it, Brehm-Stecher said.

Salmonella can be found on produce such as tomatoes, cilantro, peppers, spinach. The produce can be contaminated while it is in the fields or during processing. Washing the produce thoroughly can help, but cannot ensure that it will be safe.

The tape-FISH technique also can be used to test produce that is not suspected of being contaminated, but the volume of produce that would need to be tested may make this impractical. However, 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, Brehm-Stecher said.

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, so he began researching the idea with Bisha. Together they applied the method to produce and made several improvements in speed and sensitivity over the existing tape-FISH approach. Brehm-Stecher hopes that the technique will help speed investigations of produce contamination, such as last summer’s outbreak of Salmonella saintpaul, which was traced to imported jalapeno and Serrano peppers.

For more information, visit: www.iastate.edu