Searching for Salmonella

Date posted: August 12, 2013

Pictrue of Diane Ducharme sampling tomatoes.Dave Caldwell photoDiane Ducharme, an N.C. State University graduate student and N.C. Cooperative Extension associate, is part of a team studying how tomatoes can become contaminated with Salmonella.

A team of North Carolina State University scientists is looking for Salmonella on tomatoes and around tomato production areas. What they find could help farmers grow tomatoes that have a decreased likelihood of carrying Salmonella.

Salmonella is, of course, a bacterium that can contaminate food and make people who eat that food sick. Salmonella causes salmonellosis, a form of food poisoning. Healthy adults infected with Salmonella typically experience several days of diarrhea, fever, vomiting and abdominal cramps — unpleasant but survivable. However, for the very young, very old and people with compromised immune systems, salmonellosis can be severe and occasionally result in death.

People can come in contact with Salmonella in many ways, but the N.C. State team is most interested in how and when Salmonella contaminates food, specifically tomatoes. Tomatoes are important because they have been linked to salmonellosis outbreaks for more than a decade.

Like many types of bacteria, Salmonella is most often associated with fecal material, and fecal material is present in the natural environment, associated with wild and domestic animals, including reptiles and birds, even humans.

The problem is, even though we know Salmonella is associated with fecal material, we don’t know where in the agricultural production environment Salmonella is, which is why scientists in N.C. State’s College of Agriculture and Life Sciences are looking for it.

The N.C. State team is made up of Dr. Chris Gunter, assistant professor of horticultural science; Dr. Lee-Ann Jaykus, William Neal Reynolds Distinguished professor of food science; Dr. Otto “Chip” Simmons III, research assistant professor of biological and agricultural engineering; Dr. Penelope Perkins-Veazie, professor of horticultural science; and Diane Ducharme, a graduate student working toward a doctorate in horticultural science who is also a North Carolina Cooperative Extension associate and coordinator of a GAPs (Good Agricultural Practices) program located in the Plants for Human Health Institute on the North Carolina Research Campus in Kannapolis.

N.C. State team members bring to the table expertise in food safety (Jaykus), vegetable production (Gunter), environmental microbiology (Simmons) and postharvest physiology (Perkins-Veazie).

The team is just beginning the second year of a three-year collaborative study that partners with the U.S. Food and Drug Administration to better understand the relationship between Salmonella, potential environmental reservoirs for the bacteria and commercial tomato production.

The study is focused on tomatoes and Salmonella in North Carolina even though there’s never been an outbreak of salmonellosis associated with commercially grown tomatoes in the state, says Simmons. There have, however, been outbreaks associated with tomatoes grown in nearby states, such as Virginia and Florida.

What is curious about this is that North Carolina and Virginia are similar in many ways, so why has North Carolina been spared outbreaks of Salmonella associated with tomatoes? That’s one of the things the researchers hope to better understand.

“The purpose of the study is to locate environmental reservoirs of Salmonella,” says Gunter. “These bacteria can exist in the environment. We want to know where.”

Gunter also points out that not all Salmonella are created equal. Some types of the bacteria are more virulent than others; they may cause disease at lower doses, or they may cause more debilitating disease. The scientists want to know what types of Salmonella are in the environment.

“Without knowing where those reservoirs may be, we have no way to manage Salmonella,” Gunter adds.

Much of the search for Salmonella rests with Ducharme. While she works with growers of tomatoes and other crops regularly in her role as a Cooperative Extension associate and statewide Good Agricultural Practices (GAPs) coordinator, it is in her role as a graduate student that Ducharme is involved in the Salmonella study.

During the growing season, Ducharme regularly collects samples from three tomato production areas in different parts of the state. She collects tomatoes, obviously, but she also collects leaves and blossoms from tomato plants, soil and any fecal matter that might be in the area, such as goose droppings.

And Ducharme collects water and sediment samples from two streams and a pond used for irrigation in the tomato production areas where she samples. Those water samples may be particularly important.

Gunter points out that the FDA has identified surface water as a potential risk for contaminating crops with Salmonella. Surface water is open to the environment and may be prone to fecal contamination. It’s used in crop production for things like planting, pesticide application, irrigation and evaporative cooling. Surface water, if it is contaminated, would be a good vehicle for transporting a pathogen like Salmonella to tomato plants.

Surface water quality is typically determined by the presence of E. coli, a bacterium that, like Salmonella, is associated with fecal contamination. Because of this, E. coli is sometimes used to indicate the potential presence of pathogens like Salmonella. One of the goals of the study, Simmons says, is to determine if E. coli really is a reliable indicator of water quality and the possible presence of Salmonella.

Information that indicates how best to ensure the quality of water used in crop production could be particularly helpful to farmers as they work toward implementing new requirements of the federal Food Safety Modernization Act (FSMA).

FSMA broadens the power of the FDA to regulate food production, and the FDA’s focus is shifting toward developing food production rules. This means that farmers are going to have an increased responsibility for assuring the safety of the food they produce and will be required to take steps during the food production process to reduce the likelihood of contamination.

Farmers should benefit from the study if, as intended, it reveals some of Salmonella’s secrets. Rather than requiring a range of actions on the part of farmers that may or may not actually reduce the possibility of Salmonella contamination, the FDA should be able to use study results to focus crop production rules on actions that research indicates will more effectively reduce the chance of contamination.

“We’re really looking at the ecology of this pathogen [Salmonella] in the environment,” says Ducharme. “The tomato just happens to be the model that we’re using. But we’re looking at production practices used for many crops.”

Gunter adds, “Ultimately, we hope that we will gain a better understanding of where that pathogen exists, how it moves into crop production areas and then how to mitigate any risk that may be associated with the pathogen.”

Simmons says, “We hope to use the information that we gain through this study to develop recommendations for farmers that will reduce risks from Salmonella and other microbial contaminants that may occur naturally in commercial agriculture production environments.”

– Dave Caldwell, 919-513-3127 or dave_caldwell@ncsu.edu

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