Lettuce is believed to be responsible for four E. coli outbreaks in southwestern Ontario that made about two dozen people sick, food inspectors say.

"Romaine lettuce is the prime suspect," Rene Cardinal, an official with the Canadian Food Inspection Agency, told the Canadian Press.

Doug Sider, Niagara Region's associate medical officer of health, said the romaine lettuce was the only thing that was significantly related to the cases.

Officials will begin tracing suppliers and distributors of the lettuce if that link is confirmed through further tests.

People could also have fallen sick after eating raw vegetables prepared on the same cutting board as the source of the contamination, said Monir Taha, the associate medical officer of health for Halton Region.

Health officials still don't know the source of last month's unrelated E. coli outbreak in North Bay.

There are 128 confirmed or probable cases of E. coli 0157:H7 in the Niagara, Halton, Guelph and Waterloo regions. Of those, there are 14 confirmed cases in Niagara, five each in Halton and Guelph and two in Waterloo.

Classroom closes

Meanwhile, as part of an outbreak of a different strain of E. coli in North Bay, local public health officials closed a classroom as a precautionary measure on Wednesday.

Symptoms of E. coli are currently affecting four children from one classroom at Sunset Park Public School, said Dr. Catherine Whiting, medical officer of health for the North Bay region.

Children in the senior kindergarten class of the school's French immersion program were asked to stay at home on Wednesday. The affected classroom and common areas will be thoroughly cleaned.

A Harvey's restaurant linked to North Bay's E. coli outbreak in North Bay was set to reopen Wednesday after all testing of food and environmental samples came back negative for contamination. The restaurant has been thoroughly sanitized, public health inspectors said.

As of Tuesday, there were 251 people with confirmed, probable or suspected cases of E. coli 0157:H7. Of those, 50 have been confirmed.

There are about 350 cases of E. coli O157:H7 each year in Ontario, according to the province's ministry of health.

With files from the Canadian Press