Science - June 23, 2011

Unusual suspects

Text:
Broer Scholtens

This time it was not meat but vegetables that were the culprit. The deadly EHEC bacterium spread via a batch of mung bean sprouts. So what makes sprouting vegetables a public health danger?

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They are sure at last. After six weeks of investigations, the German authorities announced last week that the chief culprit in the EHEC epidemic had been identified. In a joint statement, federal government organizations pinpointed sprouting vegetables such as mung bean sprouts as the source of the bacteria. The suspect sprouts, which were infected with a deadly E. coli bacterium, came from an organic farm near the village of Bienenb├╝ttel in Lower Saxony.
This finding ended weeks of research by the Robert Koch institute in Berlin, the German counterpart of the Dutch National Institute for Public Health and Environment (RIVM). The investigation came in for a lot of criticism, not least from the horticulture sector, which suffered from the advice that was issued to stop eating cucumbers, tomatoes and lettuce. This advice was withdrawn in a few words in last week's press release, but it had already caused losses of at least 100 million euros for German and Dutch horticulturalists.
Since early May, 36 people have died in Germany due to EHEC bacteria on mung bean sprouts. The bacterium concerned, a strain of E coli, attaches itself firmly to cells in the human intestine, where it excretes a toxin that destroys blood vessels, particularly in the kidneys, the pancreas and the brain. And the bacterium is resistant to antibiotics.
One thousand people became infected, most of them suffering from severe and often bloody diarrhoea. Eight hundred of the victims developed haemolytic uremic syndrome (HUS), which often leads to permanent kidney or brain damage.

Infection through seeds
E. coli inhabit the intestines of mammals such as cattle, whose innards form the ideal habitat for the bacteria to multiply. The temperature is perfect and there is a constant supply of nutrients. 'In one milligram of droppings there are estimated to be several hundred E. coli  bacteria', says Marcel Zwietering, professor of Food Microbiology at Wageningen UR. 'Most E. coli bacteria are harmless. Very occasionally there are strains with pathogenic characteristics for humans, but not for animals. Sometimes, after an exchange of genes between different bacteria, a specific pathogenic strain emerges such as that 0104 strain in Germany now. And you can fall ill from just a few dozen bacteria.
There are various routes by which E. coli bacteria can spread from animal manure to mung bean sprouts. Provisionally, the German authorities are focusing of the seed route, as the specific E. coli strain has been found not only on mung bean sprouts at one particular organic farm, but also on sprouts that people grew at home. It is thought that the virulent 0104 strain came from the same imported seed.
Seeds are kept damp during germination by spraying them with water, sometimes surface water which may contain infected manure. The route to the human gut is then short: the husk of the seed is eaten with the sprout, often as an attractive garnish on salads or sandwiches. Zwietering: 'Bacteria do sometimes die out in dry conditions, but that goes very slowly, as we discovered from research we did on infected dried milk powder, among other things.' Zwietering's colleague Ernst Woltering, professor of Horticultural Chains at Wageningen University, emphasizes that growers are aware of the possible seed route. 'They take it into account. For that reason, the growers' protocols require batches of seeds to be randomly tested for the presence of E. coli  bacteria. Those protocols are intended to keep things clean. For instance, it is customary to disinfect seeds before using them.'

Source unknown
It is not surprising that it is precisely on sprouts that E. coli  are found, says horticulture expert Woltering. 'Tomatoes and cucumbers do not come in contact with irrigation water, they grow in a dry environment and get their water with nutrients through their roots, which grow in Rockwool. The method for growing sprouts is totally different, and the product is in contact with water.'
It is different for certain types of sprouting vegetable, such as mung bean sprouts, explains Woltering. The seeds and therefore also the product are directly sprayed with warm water so that they produce long edible sprouts. But tap water is used for this, not surface water. So he thinks it may have worked differently on the German farm.
Bacteria on sprouts or in seeds can spread fast. Woltering: 'The sprouts are all mixed up on a circulating turntable, so they are in constant contact with each other. That makes the exchange of bacteria easy, in what is also a warm, damp environment where they thrive. Sprouts are also delicate and because they are easily damaged, bacteria can easily get a foothold on them.'
Woltering doubts whether the exact route can ever be reconstructed now. 'There have been a few comparable incidences of infection with the best-known 0157 variant, in Japan among other places. That was on radish, which is another sprouting vegetable. The source was never established precisely then either.'
On the trail of EHEC
Hard evidence for the role of infected sprouts in the German EHEC epidemic comes from a restaurant study by epidemiologists at the Berlin Robert Koch Institute (RKI). The study was conducted in a restaurant where many of the victims had eaten. The researchers managed to track down a large group of patrons of the restaurant and ask 112 people what they ate there. The feared EHEC-0104 bacterium was found in 19 of these people.The researchers closely examined what people had eaten, studying the recipes and ingredient lists. They also looked at photos that some of the eaters had taken of their food. Those who had eaten bean sprouts turned out to stand nine times more chance of a serious infection than those who had not eaten this vegetable.
The restaurant study was the culmination of a weeks of interviews with the victims, initially over the phone and then in person. The Robert Koch Institute first gave infected people a list of open, broadly formulated questions, to get an initial overview. 'When you do this you do not usually look only at food and drink', says Dr. Yvonne van Duynhoven, epidemiologist at the RIVM. 'Other potential shared contact factors are discussed too, such as possible contact with surface water, swimming water or animals. And then researchers are usually interested in whether people have attended events or travelled.'
From previous studies in Germany it became clear that the infected people had eaten more tomatoes, cucumbers and lettuce than those who were not infected. The 0104 strain could not now be found on the suspect vegetables, however. From the later, more detailed, restaurant study, a statistically significant offender emerged: mung bean sprouts from an organic grower in Lower Saxony.

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