Study Finds that Salmonella Can Enter Tomatoes Through Leaves at VERY High Concentrations

From Food Safety News November 11

Salmonella Can Enter Tomatoes Through Leaves
By Gretchen Goetz

Recently scientists have been exploring whether or not pathogens can enter fruits and vegetables through plant parts, and have found that bacteria can indeed be taken in through the roots. Now new research shows that the leaves of tomato plants are a possible point of entry for Salmonella.

A study from the University of Florida, released Wednesday by PloS ONE publications, revealed that after leaves of tomato plants were exposed to high concentrations of Salmonella, the bacteria traveled through the plant and contaminated some of the fruit.

Researchers found that 1.5 percent of tomatoes whose leaves had been dipped into a solution with a high concentration of Salmonella then tested positive for the bacteria.

However, these findings don't mean that 1 or 2 out of every 100 tomatoes in the field will be contaminated if their leaves come into contact with Salmonella. The concentrations of Salmonella used for the study were much greater than what plants would normally be exposed to, says Ariena van Bruggen, the study's lead author, a professor of plant pathology and member of UF's Emerging Pathogens Institute.

This unusually high concentration of Salmonella was necessary, says van Bruggen, in order to ensure that if the bacteria did get through to the fruit, it would be detectable among the smaller sample size of a greenhouse full of tomatoes.

"If you use a normal Salmonella concentration that you would find in the field, you would have to test say 10,000 plants or so," she explained to Food Safety News.

The take-home message, she says, is that leaf-to-tomato contamination "can happen, but the chance is low."

"That should be stressed," she notes, "so that we don't create any panic."

But the fact that such means of contamination is possible is a reminder that growers need to be careful to review safety plans, for factors such as the source of their irrigation water or wild animal encroachment, because given the quantities of tomatoes produced in America, some tomatoes in a field where thousands of leaves are exposed to Salmonella could become contaminated, notes van Bruggen.

"It's just because we consume so many tomatoes, somebody could become ill at some point," she says.

And even that small risk is reason for safety precautions. The tomato industry brings in an estimated $619 million per year, and damaged consumer confidence could have a devastating effect on producers.

In 2008 when a Salmonella outbreak was incorrectly linked to domestic tomatoes, consumers stopped buying fresh tomatoes, and growers lost an estimated $100 million.

The study also came with a kernel of good news: contaminated seeds do not seem to contaminate the plants they produce.

"Somehow seed contamination doesn't survive in the next generation," says van Bruggen, who explained that her team planted contaminated seeds harvested from the plants that acquired Salmonella through their leaves and tested the fruits they produced, finding no Salmonella.