The number of people sickened by an E. coli outbreak traced to tainted spinach rose to 109 on Sunday, as federal officials announced more brands recalling their products.

"This is unquestionably a significant outbreak in terms of E. coli," said Dr. David Acheson, chief medical officer with the Food and Drug Administration's Center for Food Safety and Applied Nutrition.

Natural Selection Foods LLC, the world's largest producer of organic produce, has been linked to the infected greens, prompting a recall of 34 brands. Those brands include the company's own labels and those of other companies that had contracts with Natural Selection, based in San Juan Bautista, Calif., to produce or package its spinach.

On Sunday, River Ranch Fresh Foods of Salinas, Calif., added to its recall spring mixes containing spinach that were sold under the labels Hy Vee, Fresh and Easy, and Farmers Market, FDA officials said. All contain spinach purchased from Natural Selection, they said.

The recalls came as the Centers for Disease Control and Prevention intervened to help investigate the outbreak, which has killed a 77-year-old Wisconsin woman, officials said.

In Ohio, state health officials said they were investigating the death of a 23-month-old girl who was sickened by E. coli to determine whether the case was related to the outbreak. The girl's mother said she often buys bagged spinach.

CDC officials said Sunday they've started an Atlanta-based emergency operations center to help state health agencies with E. coli testing. Epidemiologists are helping test spinach samples and stool samples of infected people, center spokeswoman Lola Russell said.

The center is helping when state health agencies can't perform the tests or when a second opinion is needed, Russell said.

E. coli cases linked to tainted spinach have been reported in 19 states, with Wisconsin reporting the most.

Other states reporting cases were California, Connecticut, Idaho, Indiana, Kentucky, Maine, Michigan, Minnesota, New Mexico, Nevada, New York, Ohio, Oregon, Pennsylvania, Utah, Virginia, Washington and Wyoming, according to the CDC. The seven new cases reported Sunday were in states with previous illnesses, Acheson said.

The Food and Drug Administration continued to warn consumers not to eat fresh spinach or products containing fresh spinach until further notice.

The investigation by the FDA and the California Department of Health Services will widen Monday with the aim of tracing the spinach to individual farms, Acheson said.

The inquiry will review irrigation methods, harvest conditions and other practices at farms possibly involved.

The spinach could have been contaminated in the field or during processing. About 74 percent of the fresh market spinach grown in the U.S. comes from California, according to the California Farm Bureau Federation.

There is no indication that the outbreak was deliberate, Acheson said. It was unclear when it would be safe for consumers to start buying fresh spinach, he added.

Natural Selection recalled its packaged spinach throughout the United States, Canada and Mexico as a precaution after federal health officials said some of those hospitalized reported eating brands of prepackaged spinach distributed by the company.

However, some restaurants and retailers may be taking spinach out of bags before selling it, so consumers shouldn't buy it at all, the FDA said.

Boiling contaminated spinach can kill the bacteria, but washing won't eliminate it, the CDC warned.

Federal officials stressed that the bacteria had not been isolated in products sold by Natural Selection, best known for the Earthbound Farm brand. As the investigation continues, other brands may be implicated, officials said.

Natural Selection officials have said they're working with federal and state health inspectors to pinpoint the source of the contamination.

The company was founded in 1984 by Drew and Myra Goodman. Within two years, Earthbound Farm began shipping pre-washed, packaged salad fixings, and the company's "spring mix" became a mainstay of restaurants and supermarkets.