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The European E. coli outbreak is now the deadliest on record, with at least 22 deaths and more than 600 people in intensive care. The source of the bacteria is still unknown. Officials initially suspected Spanish cucumbers, then over the weekend pointed to sprouts from a German farm. But 23 of 40 samples from the farm show no evidence of E. coli. The unusually deadly E. coli strain, called O104:H4, has sickened more adults than children, a divergence from patterns in other E. coli outbreaks. It’s not clear why this particular strain has mostly sickened adults, but like other strains linked to adults, the bacteria are negative for a gene called eae, which codes for a gut adhesion protein, Nature reports.