STORY: McDonald’s said Sunday that beef patties in its Quarter Pounder burgers aren’t the source of the E. coli outbreak which has killed one and sickened about 75.

The fast food chain says it's certain that any contaminated food has been removed from its supply chain, and is no longer in restaurants.

The Colorado Department of Agriculture said all samples of McDonald’s beef patties tested negative for E. coli.

They added that beef testing is done, and they don’t expect more samples.

U.S. fast-food chains have removed fresh onions from their menu after they were identified as the likely source of the outbreak.

E. coli is killed in beef when cooked properly, but the McDonald’s Quarter Pounder includes raw, sliced onions.

Affected restaurants are now serving the burgers without those onions.

Since the incident, McDonald’s has removed the Quarter Pounder from about 20% of its U.S. restaurants.

Previous E. coli outbreaks have hurt sales at big fast-food chains as customers stay away from affected outlets.

McDonald’s said it would soon start serving Quarter Pounders again, and they should be in all restaurants over the coming week.