Did you know Listeria can survive on the food in your freezer?

Did you know that unlike other common harmful bacteria, Listeria can not only survive but thrive in the chilly environment of your freezer?

There have been several frozen food recalls in Canada in recent weeks (here, here, here, here, full list here) due to concerns over possible Listeria monocytogenes contamination.

Rick Holley, a University of Manitoba food safety expert, said that's one reason why Listeria is far more dangerous than contaminants such as E. coli or salmonella.

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"This little organism can grow at refrigerator temperatures, so if there's any moisture at all around on fresh produce, the organism can reach numbers that can cause older folks and people who have immune system malfunctions [to get sick]," Holley said.

Fridges can act as incubators for the bacteria, whereas E. coli and salmonella often die off in the cold, Holley said.

The usual threshold that makes people sick is 3,000 or more Listeria bacteria per gram of food, Holley said. Left to grow in a fridge, and the number of bacteria can cross the threshold, leaving pregnant women, seniors and those with chronic health issues vulnerable to serious cases of listeriosis.

More lethal than other bacteria

The mortality rate for Listeria-borne illnesses is also "unusually high" compared to E. coli or salmonella, Holley said.

The bacteria is ubiquitous — "It's everywhere in very low numbers," Holley said. It can be found in soil or grass in small quantities that often don't pose much of a health risk.

But when it builds up and becomes concentrated on equipment in food processing plants, "where things are not ideally clean," foods that come into contact with Listeria can be shipped from one factory to fridges across the country fairly quickly.

"It can become dominant and be shed into the food," he said. "Sometimes, certain areas in some of these plants can build up these bacteria."

That's more or less what happened in 2008, when 20 people died after a Listeria outbreak was linked backed to contaminated processed meats from Toronto. Another 36 cases of Listeria-related illnesses were reported in connection with the outbreak, Health Canada reported.

• 2008 Listeriosis outbreak timeline
• How Maple Leaf Foods is handling the Listeria outbreak

While it's resilient enough to stay alive in the cold, the bacteria is usually killed when cooked at "lethal" temperatures at or above 71 C, Holley said.

A full list of product recalls is available on the Canadian Food Inspection Agency's website.