A microbiologist holds a bag of tomatoes being tested for salmonella bacteria at the U.S. Food and Drug Administration's southwest regional research lab, in Irvine, Calif., on Monday, where microbiologists are working to trace the source of an outbreak. (Kevork Djansezian/Associated Press)Several restaurant chains across Canada are pulling fresh tomatoes from their menus over concerns about a salmonella outbreak in the United States.

There have been 145 cases of salmonella poisoning in the U.S. since mid-April. No deaths have been reported, but 23 people have been hospitalized.

The U.S. Food and Drug Administration said the Saintpaul strain of the bacteria comes from tainted red Roma or round red tomatoes.

Yum Brands' Taco Bell, KFC and Pizza Hut restaurants pulled tomatoes from their stores as a precautionary measure Monday.

Cara Foods, which owns Milestones Grill and Bar, Montana's Cookhouse, Harvey's and Kelsey's restaurants, pulled tomatoes from its locations on Sunday.

"There was a possibility that one of our venders was dealing with this product from this region, so we took immediate steps to pull the product out of our restaurants," said Rick McNabb with Cara Foods.

The Canadian Food Inspection Agency said there is no indication the outbreak has entered Canada, but it is watching the situation closely.

"Our alert is for consumers to be aware of what is happening in the United States … and what products they are targeting at this point. We just wanted Canadians to be informed and to make their own appropriate decisions," said Garfield Balsom, a spokesperson for the Canadian Food Inspection Agency.

Balsom said the source of the contamination has yet to be found, but once it has, if it is found the product has entered Canada, it will be removed from stores and restaurants.

Salmonella is a bacteria that lives in the intestinal tracts of humans and other animals. It usually is transmitted to humans by eating food contaminated with animal feces. Symptoms include fever and diarrhea.