Investigators are eyeing the possibility that onions, a popular topping for McDonald’s quarter pounders, could be the cause of contamination.
The CDC and FDA said on Tuesday that they had not yet ruled out that the patties themselves could be to blame.
The chain has agreed with that assessment, though the company has said that its burgers are cooked to 175 degrees – which is above the 160 degree level needed to kill the E. coli bacteria
The cases involve purchases from multiple stores, making it unlikely that food preparation is to blame, McDonald’s said.
The company said the stores involved had used multiple suppliers for the beef patties, but shared a single supplier of onions – identified as California-based Taylor Farms, one of the world’s largest vegetable processors.
Taylor Farms, which works with major food suppliers such as US Foods, has issued its own recall of some batches of onions out of an abundance of caution.
Taylor Farms said in a statement provided to CBS News that it conducted tests on “raw and finished” onion products and “found no traces of E. coli”.
On Thursday, the fast food firm Yum! Brands said it was monitoring the outbreak and had decided to proactively remove fresh onions from “select Taco Bell, Pizza Hut and KFC restaurants” in the US.
The company declined to say how many locations were affected by the decision.