COOKEVILLE, Tenn. (June 4, 2008) — Think of nutritious summer food choices, and crisp, fresh leafy greens may be the first thing that comes to mind — but if your salad makes you sick, that meal hardly fits anyone’s definition of healthy.

According to the Centers for Disease Control and Prevention, fresh produce is increasingly linked to outbreaks of food-borne illness. Currently, an outbreak of Salmonella in nine states has been potentially traced to uncooked tomatoes.

The link between fresh produce and food-borne illness is the focus of a collaborative study between researchers at Tennessee Tech University and the Agricultural Research Service’s Produce Safety and Microbiology Research Unit in Albany, Calif.

And their findings could have important implications for public health.

They’ve discovered that a particular interaction between common food-borne illness-causing bacteria and some species of microscopic organisms called protozoa — both of which are found naturally on produce — might enhance the bacteria’s ability to survive in an environment where single, free bacteria might perish.

Sharon Berk, a biology professor at TTU’s Center for the Management, Utilization and Protection of Water Resources, and graduate student Poornima Gourabathini isolated protozoa from fresh spinach and Romaine lettuce purchased at local grocery stores and fed them food-borne pathogens such as Salmonella, Listeria and E. coli O157:H7.

Those three pathogens combined result in about 115,500 cases of food-borne illness and 1,500 deaths per year, according to statistics from the CDC.

“Romaine lettuce heads and bundled spinach, unbagged, were purchased from two supermarkets, placed in plastic bags from the produce section of the stores and immediately taken to the laboratory for isolation of protozoa,” Berk said.

The research — funded by a $400,000 grant from the United States Department of Agriculture — found that the protozoa, after feeding, expelled pellets containing concentrations of live bacteria.

Although significant differences were observed in the interactions among the various bacteria and protozoa combinations, one species of protozoa produced pellets with all of the bacterial strains on which it fed.

Some pellets contained an average of 25 still living E. coli bacteria, she said.

“To determine if those bacteria were still alive, we treated them with a chemical that allowed them to grow but not divide and then fed them diluted, filtered spinach juice, to replicate the food source from their natural environment,” she continued.

Not only did the bacteria grow, but after about four hours in the spinach juice, they had moved completely free from the pellet in which they had been contained.

Those findings — included in Gourabathini’s thesis and published in the April issue of the journal of Applied and Environmental Microbiology — could have implications that range from potentially altering the way grocery stores package and display produce to possibly influencing changes in food industry disinfection guidelines.

“At the very least, it means the number of bacteria present on produce are likely to be drastically underestimated,” Berk said.

Those estimates are based on cultures of colony-forming units, and each pellet could possibly form a colony.

“If the pellet is the colony-forming unit, then that colony’s origin is not from a single bacterium. That colony would have originated from 25 or more bacteria,” she said.

That’s a significant difference, but that finding could be an initial step in the farm-to-table ecology of providing consumers with produce that has a longer shelf life and reduced risk for food-borne illness.

Ultimately, that means even healthier summer salads for everyone.

--Tracey Hackett
This information posted 5 June 2008