US fast-food chain McDonald’s is to resume selling its Quarter Pounder beefburgers following an outbreak of E coli in several states attributed to sliced onion served with the burgers. McDonald’s said in a statement issued on Sunday that it had stopped distributing the beef patties, which weigh 4 ounces (113 grams) to the impacted area “out of an abundance of caution.” It said that it had now been informed that the Colorado Department of Agriculture had completed testing confirming no detection of E coli in the samples taken and that no further testing was required. “The issue appears to be co…