(Reuters) - The U.S. Centers for Disease Control and Prevention said on Wednesday that slivered onions served on McDonald's Quarter Pounder hamburgers and other menu items were the likely source of an E. coli outbreak that sickened 90 people.

The outbreak linked to Quarter Pounder was first reported on Oct. 22, and slivered onions were suspected to be the source of the infections.

The U.S. Food and Drug Administration and the company have confirmed that Taylor Farms was the supplier for the affected locations, and it has since recalled several batches of yellow onions produced in a Colorado facility.

The FDA on Wednesday said it had initiated inspections at a Taylor Farms processing center in Colorado, a state where 29 people have fallen ill due to the outbreak.

An onion grower of interest in Washington state is also being investigated, the FDA added.

The CDC said the number of infected people has risen by 15 people from 75 and 27 persons have been hospitalized due to the illness, which has already killed one person.

On Sunday, the company along with the Colorado Department of Agriculture also ruled out the possibility of beef patties being a source of the outbreak.

The E. coli O157:H7 strain that led to the McDonald's outbreak is said to cause "very serious disease," especially for the elderly, children and people who are immunocompromised.

The FDA noted that symptoms begin anywhere from a few days after consuming contaminated food or up to nine days later.

As of Oct. 30, the outbreak affected Colorado, Kansas, Utah, Wyoming, Iowa, Missouri, Montana, Nebraska, New Mexico, Oregon, Wisconsin, Washington and Michigan.

"To say the least, the more these cases arise in the news the harder it will be for McDonald's to reset," said Jim Sanderson, analyst with Northcoast Research.

McDonald's said that it would resume selling Quarter Pounders burgers this week after it temporarily took the item off the menu in a fifth of its 14,000 U.S. restaurants that were impacted.

On Tuesday, company executives brushed off any potential sales hit from the outbreak, with CEO Chris Kempczinski apologizing to customers and adding that he was "confident in the safety of eating at McDonald's."

(Reporting by Ananya Mariam Rajesh and Aishwarya Venugopal in Bengaluru; Editing by Sriraj Kalluvila, Shailesh Kuber and Anil D'Silva)