I’m a food safety expert — these grocery store staples pose the biggest E. coli risks

This will make your stomach churn.

As dozens of people are sickened by E. coli in an outbreak linked to the McDonald’s Quarter Pounder, food safety experts are revealing what items to steer clear of to avoid food-borne illness at the grocery store.

At the top of the list is bagged, pre-washed greens.

Barbara Kowalcyk, director of Institute for Food Safety and Nutrition Security at George Washington University, told NBC News that produce that claims to be pre-washed — such as lettuce, vegetables or even cut fruit — should be avoided, as it can be a breeding ground for bacteria.

Instead, she recommends buying whole heads of lettuce and washing it herself with water and patting it dry with paper towels.

“Any leaf that is torn or damaged, I just throw it away, because that’s how the bacteria can get in there,” she explained, adding that she avoids sprouts and cantaloupe, too. “And I usually take the outer leaves off and discard them.”

Cantaloupe’s webbed rind can trap bacteria, added Darin Detwiler, a Northeastern University professor of food regulatory policy.

“It’s one of the most deadly pieces of produce out there, of all things,” he told NBC News. “Cantaloupe has a perfect pH, and you can’t clean the outside enough.”

Don Schaffner, a food science professor at Rutgers University, warned that most fruit — especially if it has been sitting out for long periods of time — also poses a risk.

“I do a lot of workshops, and they often have cut fruit on the breakfast buffet, and that fruit just sits there all day long,” he told NBC News. “We’ve done research to show that the physical appearance of it doesn’t change, but it can grow bacteria like nobody’s business.”

According to the Centers for Disease Control and Prevention, food poisoning symptoms from bacteria like listeria or E. coli — which cause infections in the gastrointestinal tract — include nausea, vomiting, fever, abdominal pain and diarrhea. Some cases can even result in hospitalization or be fatal.

Already, the McDonald’s E. coli outbreak has killed one person in Colorado and spread to multiple states, as the fast food chain’s supplier recalled yellow onions “out of an abundance of caution,” prompting other restaurants to do the same.

Schaffner explained that the more food is processed from the farm to your table, the more risk there is for it to be introduced to bacteria, which can then make you sick.

“The more manipulation you do, certainly the more places there are for things to go wrong,” he said.