What’s Happening in Salmonella Research

From the greenhouse to poultry facilities, from the classroom to 4-H camps, current Salmonella research is devoted to plants, animals, and youth food safety education.

Focus on Tomatoes

While most salmonellosis cases occur due to the consumption of contaminated poultry products, the power of fresh produce, especially tomatoes, to cause salmonellosis cannot be overlooked, says Gireesh Rajashekara, DVM, PhD, a professor in the Food Animal Health Research Program at the Ohio Agricultural Research and Development Center in the Department of Veterinary Preventive Medicine at the Ohio State University, Wooster, Ohio.

“Tomatoes have been frequently associated with wide-scale salmonellosis outbreaks worldwide over the past decades,” Dr. Rajashekara points out. “More than dozen outbreaks have been reported since 2001 in the U.S.”

The sources of these outbreaks in many instances remain unknown, Dr. Rajashekara notes. “The Salmonella infections of tomato plants and fruits do not cause any symptoms,” he relates. “The common practice of washing does not remove Salmonella, as it can internalize and avoid exposure to commonly used surface sanitizers.”

Courtesy of a $500,000 grant provided by the USDA National Institute for Food and Agriculture (NIFA), Dr. Rajashekara is in the final year of a research project focused on Salmonella contamination of tomatoes.

“Our overall goal is to understand plant-Salmonella interactions, identify environmental and biological factors that influence Salmonella persistence in tomato plants and fruits, and identify novel effective strategies to control Salmonella in produce,” he explains. “The long-term goal is to significantly improve the safety and sustainability of the tomato industry by reducing the spread of high-risk human pathogens.”

The specific objectives of Dr. Rajashekara and his research team are: 1) to illuminate both how contamination of the tomato plant with Salmonella occurs and how pathogens survive or proliferate in plant tissues and fruits, and 2) to develop novel effective control methods to prevent Salmonella contamination of tomato plants and fruits.

“We are specifically interested in understanding the environmental factors and phytopathogen infections on the survival of Salmonella in tomato plants and fruits,” Dr. Rajashekara says.

By growing Salmonella-contaminated tomato plants in green house conditions, Dr. Rajashekara and his colleagues, which include plant pathologist Sally Miller, PhD, and doctoral student Loïc Deblais, MS, observed that specific environmental temperature and relative humidity conditions have significant impact on Salmonella persistence in contaminated tomato plants.

“High environmental temperature, greater than 77 degrees Fahrenheit, significantly reduces Salmonella abundance and persistence over time on the surface of the tomato plants, however environmental temperatures did not affect internalized Salmonella,” Dr. Rajashekara explains. “On the other hand, low relative humidity levels, less than 40 percent, increased the probability of dissemination of Salmonella in the plant. Similarly, when tomato plants were experimentally infected with plant pathogens, we observed that the plant pathogens could increase the abundance and persistence of Salmonella in tomato plant tissues, which is most likely due to competition for available nutrients in the plant tissues. Salmonella once infected seems to compete successfully with plant pathogens to survive in tomato plants.”

These tomato plant studies are facilitated by the use of highly sensitive real-time bioluminescent imaging, real-time in planta imaging, Dr. Rajashekara says. “This technique allows visualizing, in a non-destructive way, the exact location of Salmonella in tomato plant and fruit tissues,” he notes, “thus providing a greater understanding of Salmonella survival and persistence in different plant tissues. This technique also allows rapid assessment of the effect of sanitizers or disinfectants on Salmonella survival and persistence in tomato plant tissues and fruits.”

Big Work with Small Molecules

Another goal in Dr. Rajashekara’s lab is to develop novel antimicrobials to control Salmonella using new generation small molecules. “Previous studies have shown that new generation small molecules are effective even against multi-drug resistant pathogens,” Dr. Rajashekara relates. “We identified several novel anti-Salmonella compounds that are effective against even the internalized Salmonella in tomato plants and fruits. These small molecules are compatible with the use of alternative control methods such as beneficial plant associated microorganisms, so thereby can be combined with bio-control approaches to enhance Salmonella control in production systems.”