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Bean sprouts are now the prime suspect in the search for a source of the E. coli outbreak.

Reinhard Burger, head of Germany's centre for disease control said on Friday that even though no tests of the sprouts from a farm in Lower Saxony had come back positive, the epidemiological investigation of the pattern of the outbreak had produced enough evidence to draw the conclusion.

Investigators had already warned answers might be difficult to find, as it is several weeks since the potentially contaminated sprouts were grown.

But he said people who ate sprouts were nine times more likely to have bloody diarrhoea than those who had not

The institute, he added, was lifting its warning against eating cucumbers, tomatoes and lettuce, but keeping it in place for the sprouts.

Some 3,000 people have been taken ill with the German outbreak of E. coli, which involves a previously unknown strain of the bacterium.

Sufferers may develop haemolytic uraemic syndrome (HUS) where bacteria attack the kidneys and nervous system, giving them fits and often forcing them on to dialysis.

It is not the first time bean sprouts - including adzuki, alfalfa, lentils and mung beans - have been associated with a health scare.

They were linked to an outbreak of Salmonella in the UK in 2010 and .

The way they are farmed and how they are prepared in the kitchen both contain potential risks.

Bean sprouts are often used in salads. If the vegetable is contaminated then eating it raw could be a health risk.

says that to be completely safe the sprouts should be cooked thoroughly until they are steaming hot.

It also advises rinsing before cooking and not eating them after the use by date.

Bean sprouts are grown from seeds, some in steam drums at a temperature of 38C.

As the agriculture minister for Lower Saxony, Gert Lindemann, puts it, this is "ideal" breeding ground for all bacteria.

However, it is uncertain how the O104 strain of E. coli could get there in the first place. The bacterium normally lives in the gut of animals and E. coli outbreaks can start when faeces are used as fertiliser or when they contaminate water.

However, Klaus Verbeck, managing director of the farm in Germany, is reported as saying: "The salad sprouts are grown only from seeds and water, and they aren't fertilised at all. There aren't any animal fertilisers used in other areas on the farm either."

It is possible that even if tests confirm that the outbreak started at the farm, this might not be the ultimate source.

Dr Stephen Smith, a clinical microbiologist at Trinity College Dublin, said: "E. coli can stick tightly to the surface of seeds needed to make sprouts and they can lay dormant on the seeds for months, during germination the population of bugs can expand 100,000 fold.

"However, and this is probably the key to the German outbreak, the bacteria are inside the sprout tube as well as outside. Thus washing probably had no effect. The bottom line is that it is crucial to source where the seeds came from and recall any stock."