Until 1990, Salmonella was usually associated with chicken, eggs or other animal-based foods.
But between 1990 and 2006, at least a dozen multistate outbreaks of salmonellosis were traced to tomatoes, accounting for an estimated 79,600 illnesses, according to the Centers for Disease Control and Prevention.
In fact, fresh produce-related foodborne illness outbreaks have increased 20-fold from the 1970s to the 2000s, and Salmonella accounts for nearly half of them, said Gireesh Rajashekara, principal investigator of the study and a food safety researcher at Ohio State’s Ohio Agricultural Research and Development Center. OARDC is the research arm of the university’s College of Food, Agricultural, and Environmental Sciences.
Gireesh Rajashekara is studying how Salmonella gets into tomatoes and hopes to find ways to prevent it from happening. (Photo by Ken Chamberlain)
The three-year grant, from the USDA’s Agriculture and Food Research Initiative, was awarded in September before the government shutdown, Rajashekara said. He is working on the project with Sally Miller, a plant pathologist at OARDC, and Corey Nislow at the University of British Columbia.
Rajashekara hopes the research will significantly add to the knowledge base regarding how Salmonella contaminates tomatoes, a problem that can cause not only significant illness, but economic losses as well.
“A foodborne illness outbreak associated with any tomato operation anywhere can have a severe economic impact on the industry at large,” he said, primarily because it takes so long to pinpoint where the problem stems from.
Preventing such problems in the first place, he said, would be ideal.
One of the challenges with Salmonella contamination of tomatoes is that the bacterium offers no telltale signs of when it infiltrates the plant, Rajashekara said.
When contamination occurs in a way that allows the bacterium to enter the plant tissue, and the bacterium encounters an environment in which it can thrive, it’s difficult if not impossible to eradicate the contamination before fresh tomatoes are sold and consumed, he said.
“We need to find ways to prevent contamination in the first place,” he said.
The researchers are looking specifically at one type of Salmonella, S. Typhimurium.
“We are looking at physical, chemical and biological factors to look closely at what might be helping Salmonella persist in plant tissues,” Rajashekara said. “Tomatoes are not a natural host for Salmonella, so to be able to survive, the bacteria need nutrients, and the plant needs to be damaged in some way to facilitate bacterial survival.”
The researchers are using an innovative approach to tackle the problem.
They’re combining two techniques: One is the use of an in-vivo imaging system, a state-of-the-art digital device at OARDC that uses luminescence to allow researchers to witness in real time how the bacteria interact with tomato seedlings under different conditions.
In addition, they will also be drawing upon Nislow’s comprehensive genome-wide screening of Salmonella to identify what conditions or treatments could keep the bacteria from proliferating, and defining the underlying molecular pathways to understand how that works.
Rajashekara said the study has three primary aims:
- Examining the effects of humidity, temperature and plant pathogen infection on the internalization, spread and persistence of S. Typhimurium in tomato plants.
“We can use the bioluminescent imaging to actually monitor the movement of Salmonella in the tomato leaves, stems, fruits and roots,” Rajashekara said. “We’ll infect the seedlings under a number of different environmental conditions or infect them with one of two plant pathogens that we hypothesize cause the plant to make nutrients available to the Salmonella -- otherwise the bacteria wouldn’t be able to survive.”
This will allow researchers to monitor the plants once or twice a day over a period of time, depending on the need, to see if the bacteria are spreading or not spreading.
“The imager allows us to do this studying in the whole plant; if we had to do this the conventional way, we’d have to grind the plant down and do a culture to look for Salmonella,” he said.
- Another aim is to identify, using Nislow’s comprehensive screening technique, vulnerabilities in Salmonella that could be exploited. The researchers do this by creating thousands of mutations in Salmonella that are molecularly barcoded to be used in automated tests, or screens.
“When we create these mutants, we know what they’re missing in their genome, so when they react to different stressors, we use the molecular barcode tags to see what gene is affecting the reaction,” Rajashekara said.
The beauty of this type of screening is that all steps in the process have been automated, with results immediately logged into a database, he said.
- Finally, the researchers, who have already screened about 5,000 compounds, have found about 10 that inhibit the growth of Salmonella in the lab.
“We want to look at how they work on the tomato plant, on tomato seed and on the tomato itself,” Rajashekara said. “Perhaps they could be used as a spray, like a bactericide, to prevent contamination.
“This would just be a pilot project to evaluate these approaches on a small scale,” he said, but would provide a basis for larger studies in the future.