Taco BellTaco Bell has withdrawn green onions from its 5,800 outlets throughout the US after preliminary tests revealed some samples to be infected with a strong strain of E. Coli. The fast food chain, a subsidiary of Yum Brands Inc, took the measure as a precaution, pending further tests on raw fruit and vegetables that will not be concluded until early next week.

Meanwhile it has been reported that health officials are investigating a warehouse in Burlington, New Jersey, as a possible source of the infection. The warehouse ships green onions to Taco Bell restaurants. Tracing the source of an infection like this is a painstaking exercise, since each stage of the supply chain from grower to consumer must be investigated. All non-meat foods are currently under the microscope.