Salmonella in the salad - washing off impossible

Salmonella in the salad - washing off impossible

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How pathogens get into the salad

Despite all hygiene measures, food infections are still extremely widespread. The pathogens enter the human body through foods such as raw vegetables, poultry, beef and other foods. For raw vegetables and lettuce, thorough washing is therefore a prerequisite for consumption. However, a recent study shows that the pathogens can also penetrate the lettuce and are therefore not washable.

Bacteria such as salmonella, listeria and E. coli enter the human body with food and cause millions of food-borne diseases every year, reports a research team from the University of Delaware. The scientists have now found that salmonella can penetrate lettuce leaves through the tiny air pores and then settle inside the plant. So it is almost impossible to wash them off before eating.

Immune defense of the lettuce plants bypassed

In their current study, the research team led by Harsh Bais and Kali Kniel from the University of Delaware examined whether and how salmonella can penetrate a plant. The study results published in the specialist journal "Frontiers in Microbiology" show that wild salmonella strains bypass the immune defense of a lettuce plant and can get into the plants via the plant's tiny air pores, the so-called stomata.

What is the gap opening?

The stomata are small openings on the leaves that open and close naturally and are regulated by the circadian rhythm, the researchers explain. The opening of the plant is used for cooling and breathing. The research team continues to close the stomata when the plants detect threats from drought or plant bacterial pathogens.

But some pathogens can use raw force to penetrate a closed stoma, explains Bais. For example, certain mushrooms are able to do this. However, bacteria do not have the enzymes necessary for this. However, it is known that some "plant bacterial pathogens have found a way to reopen these closed stomata and to penetrate the inside of the plant," said the expert.

Salmonella get through the stomata

In the new study, it has now been proven that some strains of human pathogenic Salmonella have also found a way to reopen a closed stomata. It is an interesting discovery "how the non-host bacteria develop to bypass the plant's immune response," emphasizes Bais.

Pathogen in the plant

This also illustrates the dangers that contamination can pose before harvesting. And while the risks after the harvest tend to stem from the hygiene practices in processing and distribution, these pathogens are already in the plant during the harvest.

Disinfection and cleaning have no effect

The researchers also emphasize that treatment with water and a special disinfectant and, if necessary, ozone or ultraviolet treatment, with which surface bacteria can be killed relatively reliably, do not help against human pathogens that have already penetrated the leaf.

Irrigation risks?

Furthermore, the research team had previously investigated possible contamination of plants by irrigation methods in collaboration with researchers from the U.S. Department of Agriculture and the University of Maryland. For example, part of the water is often recovered after it has already been used to wash harvested plants. This could be a problem. The researchers report just like irrigation from watercourses and ponds.

"It has been shown that water (...) poses a potential risk of contamination," says Kali Kniel. This finding is important because the question now is how can it be achieved that irrigation is always carried out with microbiologically perfect water. According to the researchers, further risks for the penetration of human pathogens into the plants include plants that have been specially bred to increase the yield, which often comes at the expense of their own defense systems. And there is also the possibility of contamination if plants are too close to a pasture.

Risks also arise before the harvest

In view of the new findings, it becomes clear that not only the superficial contamination of the plants with possible triggers of a food infection pose a risk, but that there are also risks before the harvest. Stresses on the plant that arise here may not be able to be removed afterwards, even after thorough cleaning. It is also worrying that in the current study the lettuce plants showed no signs of disease and no externally recognizable changes when contaminated with Salmonella. (fp)

Author and source information

This text corresponds to the specifications of the medical literature, medical guidelines and current studies and has been checked by medical doctors.

Dipl. Geogr. Fabian Peters


  • University of Delaware: Sneaky salmonella finds a backdoor into plants (published June 30, 2020),
  • Nicholas Johnson, Pushpinder K. Litt, Kalmia E. Kniel, Harsh Bais: Evasion of Plant Innate Defense Response by Salmonella on Lettuce; in: Frontiers in Microbiology (published April 3, 2020),

Video: Bagged Salads and Salmonella - University of Leicester (November 2022).