Researchers at Iowa State University hope a new, quick method for salmonella detection will have real-world applications, especially in the produce industry.

The method provides a two-hour turnaround for detecting salmonella on the skin of produce using a plain piece of tape and an ultraviolet light.

Fluorescent In-Situ Hybridization (FISH) is the technical name of the process that involves adding a genetic marker to a sample, then using an ultraviolet light to look for where it has bonded. The method was developed by Byron Brehm-Stecher, food science and human nutrition assistant professor at Iowa State, and graduate student Bledar Bisha, and involves the use of tape to remove the sample of contaminants from the outside of the item being tested, hence the nickname, “tape-FISH.”

Although sterile tapes designed for use in laboratories provide the best results, the tape commonly found on desks and used to wrap packages also works.

Produce has been the focus of experiments so far, Brehm-Stecher said. Researchers have tested tomatoes, spinach, cilantro and jalapeño peppers, he said.

“The tape picks up microbes where they are on the produce surface, creating a mirror image on the tape surface of how the organisms were arranged,” Brehm-Stecher said.

This information can be valuable in preserving details of how the organisms are arranged or how they interact on produce surfaces.

“Are they located primarily around the veins of cilantro leaves or the bud scars of tomatoes, for example?” he said. “Or are there more salmonella present at the site of a mold infection? We are doing some of this work, but expect that others may also contribute to this area, using the tape-FISH approach.”

The real-world applications for this method would be on smaller samples and when it is important to know the spatial arrangement of salmonella on produce, he said.

“Searching for microbial contaminants on produce is like searching for the proverbial needle in a haystack,” Brehm-Stecher said. “Contamination levels are low and contamination may be sporadic.”

Wash-based systems that can remove microbial cells from a larger produce sample, which in turn can then be concentrated into a smaller sample might be more practical for routine surveillance than tape-FISH, he said.

“However, the tape-FISH approach may be useful to zero in on salmonella present on smaller samples,” Brehm-Stecher said.

The Iowa State University research was published in the March issue of Applied and Environmental Microbiology, an academic journal. The duo first presented information on the research project to the industry at United Fresh 2008 in Las Vegas.

“We were part of the (U.S. Department of Agriculture’s) multistate research project titled ‘Postharvest Quality and Safety in Fresh-Cut Vegetables and Fruits,’ ” he said.

The researchers have worked hard during the past year to develop the method from its initial stages in the spring of 2008.

“This was around the same time that a national outbreak of salmonella was beginning to take shape,” Brehm-Stecher said. “It was very exciting to be able to develop a potentially useful tool for detection of salmonella in these foods in the midst of an outbreak, as the importance of this work was continually reinforced every day in the news media.”

Brehm-Stecher and Bisha are talking with producers about potential field trials, and they expect those to be scheduled once funding becomes available.

“We hope the method will serve as another useful tool for rapid detection of salmonella, and that it may be applicable to other food systems, as well,” Brehm-Stecher said.

Researchers develop two-hour Salmonella test
Iowa State University scientists have developed a process using tape to detect salmonella within two hours.