Science - June 1, 2006

Danger of food poisoning from lettuce

Consumers often put a bout of food poisoning down to having eaten eggs or meat. But some cases of food poisoning are actually caused by eating raw vegetables, according to PhD researcher Eelco Franz, at the Organic Farming Systems Group. Bacteria that cause diarrhoea, like salmonella and E. coli 0157, come in contact with lettuce plants through manure and are even taken up by the plants.

Franz has done experimental research on the food safety risks associated with using animal manure in lettuce growing. He examined how easily dangerous bacteria can survive in manure. Organic vegetable growers use animal manure as their standard fertiliser, and the tightening up of regulations for manure disposal in the Netherlands means that conventional vegetable growers have also started use more animal manure. The results show that the E. coli strain in question can survive for longer than eighty days in manure. It survives longest in manure from conventional livestock, where the cattle are fed mainly maize. Survival is considerably shorter in the manure of cattle that have an organic and fibre-rich diet of ensiled grass. How long the bacteria survive in the soil also depends on the type of farming and the soil type. The dangerous bacteria live longest (sixty days) in clay soil on regular farms. In sandy soil that is organically farmed, they disappear five times as fast.

The laboratory experiments showed that contamination is not only on the outside of the plant, however. The lettuce plants absorb the bacteria and can therefore also be internally infected. According to Franz, the concentration levels – up to a thousand bacteria cells per gram of lettuce – are high enough to make a person ill after eating contaminated lettuce. What’s more, internal contamination means that washing the lettuce has no effect.

A way to reduce the chance of contamination would be to compost manure. Another measure would be to ensure that there is a period of at least four months between the moment of manure production and harvesting the vegetables. Franz emphasises that there have been no known outbreaks of food poisoning through raw vegetables so far in the Netherlands. Nevertheless, he is sure that among people who have assumed they have had food poisoning as a result of eating egg or chicken sate, there will cases where fresh vegetables were the culprit.

The Food and Consumer Product Safety Authority (VWA) is taking the danger of infection through raw vegetables seriously and is looking into ways to prevent the presence of dangerous bacteria in vegetable and fruit products. VWA project leader Paul in ’t Veld: ‘We already knew that pathogenic bacteria are found on lettuce leaves. It’s news that they are also found in the leaves, but it doesn’t really surprise me. We will be doing screening to see how serious the problem is in vegetables on the market.’ He expects to be able to present the first results of the screening in two years. / GvM