Bean sprouts from an organic farm in northern Germany caused the E coli outbreak that has killed 31 people and infected thousands more, German officials said on Friday.
Health inspectors have identified the source of the infections after linking patients who fell ill with the bug to 26 restaurants and cafes known to have received produce from the farm in Lower Saxony.
Reinhard Burger, the head of the Robert Koch Institute, which is responsible for disease control and prevention in Germany, told a press conference in Berlin there was sufficient evidence to implicate the farm, even though bean sprouts there had passed tests for the lethal microbe.
"It was possible to narrow down epidemiologically the highly probable cause of the outbreak of the illness to the consumption of sprouts," Burger said.
The breakthrough, which ends days of confusion, led German authorities to lift a warning against consumption of cucumbers, tomatoes and lettuce issued in May soon after the outbreak was detected.
The move came as Russia offered to lift a blanket ban on imports of European vegetables as soon as Brussels had provided proof that the food was safe.
The farm at the centre of the outbreak was shut down a week ago and its produce recalled. Health inspectors closed in on the grower after studying the menus and ingredients at eateries, in some cases going through bills and taking pictures of the meals to show to patients who fell ill with the infection.
Burger said that it was possible all the tainted sprouts had been eaten or thrown away, but as a precaution people should not eat sprouts.
Nearly two weeks ago health officials in Hamburg blamed Spanish cucumbers for the outbreak and then retracted the claim. The European Commission has since agreed to pay €210m in compensation to growers whose produce had to be thrown away.
Andreas Hensel, the head of Germany's risk assessment agency, said: "Lettuce, tomatoes and cucumbers should be eaten again – it is all healthy produce."
Organic farms fell under suspicion in the investigation because they do not use chemical fertilisers and put crops at greater risk of contamination from bacteria in manure. The cost to European farming could reach €500m.
The new strain of E coli causes disease by colonising the gut and producing a toxin called Shiga. Many patients experienced bloody diarrhoea and in the most serious cases the infection caused a life-threatening condition called haemolytic uraemic syndrome (HUS).
