Tomato turf wars: Benign bug beats salmonella; tomato eaters win

When applied to Salmonella-contaminated tomato plants in a field study, the bacterium, known as Paenibacillus alvei, significantly reduced the concentration of the pathogen compared to controls.

Outbreaks of Salmonella traced to raw tomatoes have sickened nearly 2,000 people in the US from 2000-2010, killing three. Since the millennium, this pathogen has caused 12 multistate outbreaks of food-borne illness -- more than one each year. It was this carnage that provided the impetus for the study, according to corresponding author Jie Zheng, of the FDA.

"The conditions in which tomatoes thrive are also the conditions in which Salmonella thrives," says coauthor Eric W. Brown, also of FDA, "but we knew that if we could block Salmonella from infecting the tomato plant, we could reduce its risk of infecting the person who eats the tomato."

The logic behind the work is simple. Many innocuous bacterial species thrive within the tomato-growing environment.

"We hypothesized that such an organism could be found that possessed the ability to outcompete or chemically destroy Salmonella," says Zheng. "After screening many hundreds of potential biocontrol strains of bacteria that were isolated from farms and natural environments in the Mid-Atlantic region, we found about 10 isolates of bacteria representing very different genera and species that could curb the growth and/or destroy Salmonella in our test assays."

Many of these were as pathogenic to humans as is Salmonella, but two isolates, belonging to the environmentally friendly species, P. alvei, strongly inhibited growth of Salmonella.

"This bacterium also has no known history of human pathology, making it a great candidate as a biological control agent," says Zheng.

"While farmers and agricultural scientists have long used microbes to prevent plant diseases, we now have the opportunity to add a naturally-occurring microbe to a crop in the field with the goal of preventing human disease," says Zheng. "Our ambitions are now to extend this microbial approach to cantaloupe, leafy greens, and other crops that have lately been responsible for outbreaks of food-borne Salmonella and E. coli."