Beilei Ge in her laboratory at the LSU AgCenter in Baton Rouge, La. (Click on photo for downloadable image.)

News Release Distributed 03/03/10

A quick way to test for salmonella in vegetable fields may be in the offing if research by an LSU AgCenter scientist proves its worth.

Working under a $40,500 grant from the Center for Produce Safety at the University of California, Davis, Beilei Ge in the Department of Food Science is developing a test for contamination by food-borne pathogens in produce bound for the retail marketplace.

Ge is evaluating a process that samples salmonella using a technique scientists call DNA amplification, which increases the volume of the DNA to a level high enough to detect easily. As one of the most common causes of food-borne disease outbreaks, salmonella is responsible for an estimated 1.4 million annual cases in the United States, according to the U.S. Centers for Disease Control and Prevention.

The process tests samples of fruits and vegetables for the presence of DNA linked to certain disease-producing microorganisms, generally referred to as pathogens. Current tests use a technique called PCR – or polymerase chain reaction – that detects salmonella based on DNA, Ge said.

LSU AgCenter researchers have been developing and evaluating a novel isothermal – one constant temperature – method to amplify certain genes. Similar to PCR, this technique starts with efficiently amplifying the target bacterial genes, duplicating the initial gene millions of times in about an hour.

“The ability to quickly and accurately detect the presence of even low levels of harmful microbes is essential for food safety and biosecurity,” Ge said. “An ideal detection method is rapid, sensitive, specific and cost-effective.”

Currently, food-borne pathogen detection relies heavily on conventional microbiological culturing techniques, which are labor-intensive and time-consuming, she said. Although newly developed molecular techniques have improved performance, they still lack sensitivity, take a long time for analysis or require expensive equipment.

The widely used PCR process measures all DNA, and the results can’t differentiate between live and dead salmonella cells so it may give false-positive results. In addition, PCR requires designated laboratory equipment that takes samples through several cycles of changing temperatures over several hours.

“PCR tests have to be sent to a lab and often return false-positive results because they can’t determine between live and dead salmonella,” Ge said.

Ge is working with a novel isothermal process which was developed in Japan and is beginning to gain momentum in the United States. Her LSU AgCenter lab is one of the first U.S. labs to adopt the technique.

The novel DNA amplification process Ge uses requires only one temperature – about 150 degrees – and the test can be done within an hour. While the PCR technology requires the produce sample be sent to a laboratory, Ge said, the new technology has the potential to be used in the field.

Additionally, Ge’s technique measures only DNA from live salmonella. She uses a chemical that “binds” the DNA in dead cells so it isn’t detected by the test.

“The chemical is quite specific for dead salmonella cells but not live ones,” she said.

By combining these two elements – the novel DNA amplification method and a novel chemical compound – Ge’s approach will be able to develop a new way to identify salmonella in produce.

“The test results are visual,” Ge said. “You can tell by the turbidity of the sample whether you have a positive result. It also reacts to a dye and gives fluorescence to identify contaminated samples.”

Ge is inoculating produce – cantaloupe, tomatoes and spinach – with salmonella and then checking how well the test can identify the pathogen. Her next step will be to test the process in production fields.

“We’re currently evaluating the process in the laboratory,” she said. “We’re looking at how sensitive and specific the results can be.”

Because a test does not require complicated equipment and sophisticated technical skill, a field test would be feasible, she said.

During her five years with the LSU AgCenter, Ge has used novel molecular detection assays to detect different bacterial pathogens. The current salmonella detection project is a collaboration with John Beaulieu, a plant physiologist with the U.S. Department of Agriculture’s Southern Regional Research Center in New Orleans.

Rick Bogren