Pathogenic bacteria on raw vegetables

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We mainly associate food safety issues with salmonella in chicken, but raw fruit and vegetables can also harbour dangerous bacteria. Researcher Leo van Overbeek is studying how pathogens infest our crops, which will help growers to prevent this. “A lot still happens in agriculture that we don’t understand.”

“We’re eating more and more fresh fruit and vegetables. At parties, I see raw cucumber, pepper and even raw cauliflower.” Microbiologist, researcher and project manager Leo van Overbeek is happy to eat it all but he also issues a warning. He studies how bacteria (such as EHEC, short for enterohaemorrhagic E. coli) get onto plants and make people sick. These pathogens are killed by cooking, but there is a risk of food infections if the plants are eaten raw.

Pathogens can easily end up on plants, as Van Overbeek explains. The pathogens are found in natural fertilizer, for example, such as cow manure. If you use this to fertilize lettuces, bacteria (including ones that make humans ill) are attracted by substances secreted by the roots. And if it rains, the spatters can splash the bacteria onto the leaves. EHEC and other pathogens can also end up on the plant if ditchwater containing bird droppings — and therefore the bacteria — is used for irrigation.

Considerable damage

To illustrate how disastrous the consequences can be, the researcher points to 2011. “There was a huge outbreak of EHEC in Hamburg. A lot of people died as a result. But we often forget that the economic impact can be severe too. The borders were closed for food exports.” There was no alternative, explains Van Overbeek, because no one knew what was causing the illness. It eventually turned out to be contaminated fenugreek, a plant that is often used as decoration on dishes in restaurants.

Pathogens can be transferred to crops via manure. Photo: Havelaar FotografieAH / Nationale Beeldbank

“But when the search began, no one had any idea. Cucumbers, tomatoes — all kinds of potential culprits were suggested. So everything was stopped and returned.” In other words, destroyed. Greenhouse growers, such as tomato growers, were particularly hard hit as they invest everything in a planned harvest and need the revenue from it to pay for next year’s greenhouse crop. “If their income stops, they go bankrupt. That’s also what happened in 2011. The economic damage from a crisis like that is considerable.”

The pathogens are killed by cooking but there is a risk of food infections if the plants are eaten raw

Fortunately, food crises due to pathogens on plants are relatively rare. “2011 is almost a decade ago and there hasn’t been such a large outbreak since. However, we have seen more minor outbreaks associated with fresh fruit and vegetables. Infections from spinach are common in the United States, there was recently an incident involving mushrooms in Canada and problems are occasionally reported with lettuce.” But as he emphasizes, “If you compare this with global production volumes, such outbreaks are a very small fraction of the total.”

How big is the risk?

To keep that percentage so low (and preferably reduce it further still), the Dutch government and food producers are funding research by Van Overbeek and his colleagues at Wageningen University & Research. They are studying how such pathogens end up on plants, in what quantities and how often this happens. “After all, you want to know how big the risk is in practice. You need a dividing line between what is acceptable and what is not. The decision about what we are willing to accept is for politicians to take, not scientists.”

We already knew that you can become sick from eating raw meat or raw eggs, but pathogenic bacteria such as E. coli or salmonella can also be found on uncooked fruit and vegetables. WUR researcher Leo van Overbeek explains why that is and what Wageningen University & Research is doing to make sure we can continue to eat lettuce and strawberries safely.

The research doesn’t just look at the plant, it encompasses the entire ecosystem: the plant’s sources of nutrients (fertilizer and water), the soil (including the soil life) and the plant itself. “The fact that I connect up these three separate ecosystems makes this research unique.” Even so, Van Overbeek sees this approach as only logical. “In agriculture, these three systems combined produce something new. All kinds of things happen that aren’t yet understood.”

Grains, lettuce and potatoes

The researcher wants to gain insight into that poorly understood terrain in order to help growers make the right choices when trying to stop the spread of human pathogens. For example, he now knows that key sources of infection are manure, and irrigation using surface water and water from basins. There are already guidelines geared to minimizing the risk but they are very broad-brush. “There is a guideline, for instance, that says when you can use surface water until, after which you should switch to piped water. Another guideline says when you can dig manure into the soil until. But there are so many different kinds of manure and each kind has its own problems. That’s why you need specific guidelines for particular crops.”

Van Overbeek cites the example of grain. “We don’t eat raw grain because we turn it into bread and beer, whereas we do eat raw lettuce. And potatoes spend longer in the field than lettuce does. So you need stricter rules for some crops than others. We need to make more of a distinction and provide knowledge that is specific to the crops that you can eat raw.”

Alternative host

What is more, professional food producers know too little about the risks. “And people with their own vegetable gardens know nothing at all about this topic.” The researcher therefore stresses the importance of effective communication about the risks associated with EHEC and other pathogens.

Vegetables such as Brussels sprouts can become infected but the risk of a food crisis is relatively small. Photo: Jan van Oevelen / Nationale Beeldbank

The microbiologist gives two reasons why this issue has not received much attention to date. Firstly, infections from fruit and vegetables are much less common than infections from animal products. Secondly, it was thought for a long time that pathogens could only survive in animals. “We thought: a plant never gets up to 37 degrees centigrade and neither does it contain the huge supply of nutrients you see in animal intestines, so those pathogens won’t be found in plants. But we now know that plants can be an alternative host for these bacteria. The risk of infection is much lower but there is a risk.”

Risks of circular agriculture

An additional complicating factor is the transition to circular agriculture. While this approach, in which all raw materials and waste products are reused as often and for as long as possible, is essentially beneficial, it does entail risks according to the researcher. “It means the system also accumulates undesirable substances, such as human pathogens, remnants of pesticides and mycotoxins, and antibiotic-resistant bacteria. The latter can end up in our intestines, where they can transfer the resistance to other microorganisms. This can result in an accumulation of resistance genes in our bodies. These risks should be considered properly, taking into account the entire supply chain and all the relevant ecosystems, not just the plant itself.”

The risk of infection from plants is much lower, but there is a risk

Has this article made you afraid? That is quite unnecessary, as Van Overbeek stresses repeatedly. “My message is simple: all the fresh fruit and vegetables in the Netherlands are perfectly safe. No question. Though let’s be clear: I can’t comment on vegetable gardens. But as far as large batches are concerned, they are checked very thoroughly by the companies that process the food, such as Albert Heijn.” Things will only improve further if the growers take the risks into account too. “That is not happening enough yet, whereas this issue needs attention at the start of the supply chain too. Growers themselves want this too. They are eager to work on food safety but they need to know what to focus on.”

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