Would you like a side of food poisoning with that salad?

Salmonella can infect plant cells and successfully evade all the defense mechanisms of plants so cleaning the surfaces of raw fruits and vegetables, e.g. by washing, is not sufficient to protect against food poisoning, according to a study published today.

The results of the project are based on a model plant, which also represents the ideal basis for future development work on treatment and testing systems in the area of food safety.

1.5 billion (!) cases of food poisoning are caused by Salmonella bacteria each year (World Health Organisation). If the bacteria survive particularly well in a person, they can even infect intestinal cells and persist for longer. Previously, the only known sources of infection were infected meat products and plants that had come into contact with contaminated water.

Unnoticed on your plate - here marked with a fluorescent protein, Salmonella in plant cells. Credit: Heribert Hirt

Work by the Unité de Recherche en Génomique Végétale (URGV) in Evry, France, and the Max F. Perutz Laboratories (MFPL) in Vienna, Austria, has now shown that this is not entirely true.

Work carried out by a team led by geneticist Prof. Heribert Hirt, and published today in PloS ONE, shows that the strain of bacteria known as Salmonella typhimurium can also invade, and multiply inside, plant cells. It is already known that Salmonella can survive for up to 900 days in contaminated soils, which creates a rich source of infection for plant material. However, Prof. Hirt's team can now show that bacteria from such a source can actively achieve the infection of plant cells, thereby disproving the previous assumption that infection was coincidental and - as regards the bacteria - passive.

Prof. Hirt explains: "We marked individual bacteria with a fluorescent protein, which enabled us to observe them as they quite clearly penetrated root cells and multiplied. Just three hours after the bacteria came into contact with the roots, they had penetrated inside the cells of the finest root hairs. 17 hours later, the cells inside of the roots had also become infected."

In principle, plants are anything other than helpless when under bacterial attack, and know how to defend themselves. They have a whole range of defence mechanisms they can use to ward off infection. The team also investigated the efficacy of these mechanisms when under attack from Salmonella bacteria.

Prof. Hirt describes their results: "The defence mechanisms fail completely. Although regulating proteins such as the two mitogen-activated protein kinases 3 and 6 are activated just 15 minutes after Salmonella has infected the plant, they cannot prevent the bacteria from multiplying. Another defence mechanism, which is activated by the plant messengers salicylic acid, jasmonic acid and ethylene, proves similarly ineffective. Although these messengers are important to coregraph the plant defense responses, they too are unable to halt the infection."

Prof. Hirt's discovery has important implications for the production and processing of foodstuffs. As emerging nations develop into industrial countries, a development that can be witnessed around the world, their needs for food and water also grow.

Besides the use of organic manures, many of which come from animals, these needs also necessitate irrigation, often with contaminated - and therefore potentially infectious - water. If, as has now been discovered, Salmonella survives and multiplies in plant cells, then washing raw fruit and vegetables does nothing to prevent food poisoning.

Instead, scientists need to develop new methods of treatment and testing to tackle Salmonella infections in plants. This FWF-supported project has already created the ideal basis for this work in the form of the model plant Arabidopsis thaliana, which was used by the team from URGV and MFPL in its study.

Article: The dark side of salad: Salmonella typhimurium overcomes the innate immune response of Arabidospis thaliana and shows an endopathogenic lifestyle. A. Schikora, A. Carreri, E. Charpentier, Heribert Hirt, Plos ONE.