McDonald’s to resume selling Quarter Pounders in all restaurants after beef patties in Colorado test negative for E. coli CNNE. coli Outbreak Linked to McDonald’s Quarter Pounders | E. coli infection CDCMcDonald's May Have Identified the Culprit of Its Massive Multi-State E. Coli Outbreak Food & WineNo E. Coli Found in Samples of McDonald’s Beef Patties, Officials Say The New York Times