Linda Benedict, Bogren, Richard C.
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 foodborne pathogens in produce bound for the retail marketplace.
Foodborne pathogen detection relies heavily on microbe-culturing techniques, which are labor-intensive and timeconsuming, Ge said. Although newly developed molecular techniques are an improvement, they still lack sensitivity, take a long time for analysis or require expensive equipment.
As one of the most common causes of foodborne 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.
Current tests use a technique called PCR – or polymerase chain reaction – that detects salmonella based on DNA amplification, which increases the volume of DNA to a level high enough to detect easily. The process tests samples of fruits and vegetables for the presence of DNA linked to certain pathogens.
Ge has been been developing and evaluating a constant-temperature process that uses DNA amplification to duplicate target bacterial genes millions of times in only about an hour compared with PCR that often takes days, including a trip to a laboratory.
“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. 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’s technique measures DNA only from live salmonella because 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 DNA amplification method and a specific chemical compound – Ge’s approach will be able to develop a new way to identify salmonella in produce.
Ge is working with a process initially developed in Japan and gaining use in the United States. Her LSU AgCenter lab is one of the first U.S. labs to adopt the technique.
The DNA amplification process Ge uses requires only one temperature – about 150 degrees – and the test can be done within an hour. Unlike the PCR technology that requires the produce sample be sent to a laboratory, Ge’s technology could potentially be used in a grower’s field.
“The test results are visual,” she 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.
During her almost six years with the LSU AgCenter, Ge has used molecular detection 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.
Besides detection research, Ge’s laboratory also is involved in surveillance studies to examine the contamination levels, genetic makeup and antibiotic-resistance profiles of foodborne pathogens in various foods. She’s also investigating genetic traits that contribute to the virulence and development of antibiotic resistance in foodborne pathogens.
(This article was published in the summer 2010 issue of Louisiana Agriculture.)