Fresh onions are the probable source of an E coli outbreak at McDonald’s restaurants that has sickened 49 people and killed one, the US Department of Agriculture said, alarming fast food chains using the ingredient. Taylor Farms, a supplier for McDonald’s, the biggest US burger chain, recalled several batches of yellow onions produced in a Colorado facility, according to a memo on Wednesday by US Foods, one of the largest suppliers of food service operations in the country. The US Foods recall alert does not mention whether the company supplies onions to McDonald’s. Fresh onions are an ingredient in the McDonald’s Quarter Pounder hamburger, and McDonald’s has pulled the item from its menu in Colorado, Kansas, Utah and Wyoming, and in parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma. Some types of E coli bacteria are harmless but a few strains cause illness and even death. Past outbreaks of the bacterial disease have hampered sales at big fast food restaurants, as customers shun the affected chains. Yum Brands said on Thursday that it was removing fresh onions from menus “out of an abundance of caution” at some of its KFC, Pizza Hut and Taco Bell chains. Restaurant Brands International, parent of Burger King, the McDonald’s rival, had removed onions from its menu at at least one outlet in Colorado, the state at the center of the McDonald’s outbreak. “We’ve been told by corporate to not use any onions going forward for the foreseeable future,” said Maria Gonzales, the on-duty manager inside a Burger King in Longmont, Colorado, on Wednesday. “They’re off our menu.” Neither McDonald’s nor Restaurant Brands International immediately responded to requests for comment on Thursday.