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Salmonella Can Enter Tomatoes through Leaves

Tim Boyer's picture

Salmonella can enter tomatoes through the leaves of an otherwise healthy, undamaged tomato plant is a recent discovery from studies looking at Salmonella contamination in food-producing plants. One reason for this may be attributable to the application of fungicides, insecticides and herbicides containing surfactants that appear to aid the colonization and the entrance of Salmonella via the leaves of tomato plants.

Salmonella is a common and pervasive foodborne illnesses generally associated with raw or undercooked meat, poultry and eggs. However, it is also sometimes present through contaminated produce from unsanitary farming practices or poor produce handling procedures. Salmonella poisoning is well-known for causing severe abdominal pain, fever, nausea, vomiting and sometimes death.
Previously, recent studies have shown that breaks in the skin of a fruit or in the stems of plants can allow Salmonella bacteria to eventually work its way into the fruit. While carefully choosing a healthy appearing, undamaged fruit at the supermarket, followed by washing the fruit at home are good measures toward preventing Salmonella poisoning, the results of this study and others like it, indicate that such measures are not a guarantee of protecting your family against Salmonella poisoning.

Researchers from the University of Florida published their tomato leaf-Salmonella findings in a recent issue of the scientific journal PLoS One. The goal of the study was to investigate whether Salmonella can enter a tomato plant’s leaves depending on the morphotype of the Salmonella bacterium and the effects of chemical surfactants used in fertilizers and fungicides. Surfactants are used to promote the spread and adhesion of chemicals to the leaf surface of plants.

In a repeated 10-month long study, a single leaf from each of 84 tested tomato plants was coated with a common surfactant used in pesticides, and then coated with a potent solution of concentrated Salmonella.

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The results of the first study showed that nine tomatoes from one plant treated with surfactant and Salmonella tested positive for Salmonella. In the repeat study, Salmonella was detected in eight tomato plants, with two plants yielding 12 tomatoes that tested positive for Salmonella. In addition, some non-inoculated plant leaves that were near the inoculated leaves also tested positive for Salmonella.

The results of the study led to two major findings: that at least one type of Salmonella can enter tomato fruits internally via the phloem tissue with Salmonella originating on the leaves externally; and, that surfactants do enhance the initial leaf colonization and possibly the entrance of Salmonella in tomatoes and other food-producing crops.

The lead researcher in the study, plant pathologist Ariena van Bruggen, a professor in the University of Florida’s Institute of Food and Agricultural Sciences states, however, that the public need not be overly alarmed by the findings because she believes that the chances of this happening in the field outside of laboratory conditions are relatively low.

However, the conclusion of their study does show that further study is needed to determine to what extent surfactants in chemicals for plant production are a risk for potential Salmonella contamination. If surfactants prove to be a significant risk factor, farming practices will need to alter their use of pesticides and fungicides for safer crop production.

Source: Gu G, Hu J, Cevallos-Cevallos JM, Richardson SM, Bartz JA, et al. (2011) Internal Colonization of Salmonella enterica Serovar Typhimurium in Tomato Plants. PLoS ONE 6(11): e27340. doi:10.1371/journal.pone.0027340
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