• Burger King and Yum Brands have pulled onions from select restaurants following an E. coli outbreak tied to McDonald's.
• It comes after restaurant supplier U.S. Foods on Wednesday issued a recall notification for four onion products produced by Taylor Farms.
• Health authorities are investigating if slivered onions served on McDonald's Quarter Pounders were the source of the outbreak.

Burger King and Yum Brands, which owns Taco Bell, have pulled onions from select restaurants following an E. coli outbreak tied to McDonald's.

"As we continue to monitor the recently reported E. coli outbreak, and out of an abundance of caution, we have proactively removed fresh onions from select Taco Bell, Pizza Hut and KFC restaurants," a Yum Brands spokesperson said in a statement to CNBC. "We will continue following supplier and regulatory guidance to ensure the ongoing safety and quality of our food."

Yum did not specify how many of its restaurants are included in the measure. It's unclear if Yum removed the onions from select locations in response to a recall related to the McDonald's outbreak.

Taylor Farms supplies McDonald's onions in the affected region and also provides products to restaurant supplier U.S. Foods. U.S. Foods, which does not supply McDonald's restaurants, issued a recall on Wednesday for four onion products produced by Taylor Farms.

Taylor Farms has not responded to CNBC's request for comment.

Burger King is removing onions from 5% of its U.S. restaurants after reviewing its supply chain and determining those onions originated at the Taylor Farms Colorado facility at the center of the recall.

The burger chain said it only uses whole, fresh onions. Its employees cut, peel, wash and slice the onions at its restaurants.

"Despite no contact from health authorities and no indications of illness, we proactively asked our 5% of restaurants who received whole onions distributed by this facility to dispose of them immediately two days ago and we are in the process of restocking them from other facilities," a Burger King spokesperson said in a statement to CNBC.

Health authorities are currently investigating the source of the E. coli outbreak, which has led to one death and 49 confirmed cases across 10 states, including Colorado, Nebraska and Wyoming. The Centers for Disease Control and Prevention has interviewed 18 people, 14 of whom remember eating a Quarter Pounder burger from McDonald's, as of Tuesday.

In response to the outbreak, McDonald's has pulled Quarter Pounders from roughly a fifth of its U.S. restaurants. The investigation has honed in on two ingredients in the burgers: the fresh beef patties and slivered onions.

McDonald's said the affected restaurants all source onions from a single facility, which washes and slices the onions. Its beef patties, on the other hand, come from multiple suppliers in the region. If cooked according to internal standards, the temperature would kill any E. coli in the patty.

— CNBC's Kate Rogers contributed reporting for this story.