Listeria: Understanding the symptoms and risk factors of the foodborne bacteria

Listeria is found in a large group of foods, including vegetables.

The CDC recommends consuming leftovers before 3-4 days after refrigerating.

Listeria might not be the most well-known bacteria, but it’s one of the most deadly – recently, two elderly Australians have died after eating contaminated smoked salmon.

And the bacteria isn’t just found in meat: Consumer Reports revealed a test of fresh greens – such as lettuce, spinach and kale – found six out of 284 samples were contaminated with listeria.

To make things worse, it’s one of the very few bacteria that can still grow in refrigerator temperatures.

“Things like salmonella and related organisms grow well at body temperature, but it typically doesn’t grow below about 10 degrees – whereas listeria still grows quite happily at four degrees,” said Julian Cox, Associate Professor of Food Microbiology at UNSW.

“One of the biggest problems with listeria is that it occurs in the industrial environment in food production.

“You’ll often find something like salmonella gets into food way back in primary production, say on the farm – but listeria can often contaminate food in processing and further processing.

“So you’ll find it in cheeses and meats as well as leafy greens, which is more of its natural environment.

“The packaged greens typically go through some kind of sanitary process designed to help destroy the organism, but we may not be totally successful in that.”

Symptoms of listeria
Unlike other common foodborne illnesses, symptoms can form up to 70 days after infection.

“In some cases, food poisoning can occur as quickly as 15 minutes – but listeria is very different. Often it takes a while to incubate in the body, and it can take two months or more to show symptoms,” Cox said.

“Symptoms vary as well – in healthy people, we may not experience any disease at all, or we might get a mild flu-like illness.

“But we can get typical gastro, and we can get complications like brain inflammation and so forth. That is much more typical in people who are susceptible – what we call a ‘YOPI’: Young, Old, Pregnant, Immuno-compromised.

“So the people who have been infected and died often have underlying medical conditions that make it worse.”

Safety tips
“If we are dealing with healthy adults, typically many of the foods are not a problem,” Cox said.

“There could be a low level of listeria in deli meats or soft cheese and that’s not a problem.

“For other people, a low level could be a problem, and removing it is almost impossible. So we have to ask those people who are susceptible to perhaps avoid those foods – that’s typically been the approach.

“I make sure that I have different cutting boards for salads and meats, and my fridge is kept at two degrees – which means even leftover foods last a lot longer.”