Listeria: Craftier Than First Believed


AUBURN, Feb. 10, 2004 --- Until now, scientists knew listeria was a serious pathogen; they just 
didn’t know how serious.  

As it turns out, listeria is one crafty pathogen – craftier than they ever imagined.  And adaptive.  
And tenacious.

Scientists have discovered the wily bacteria can grow and reproduce even in the acid conditions 
of the gall bladder.  It also can survive outside cells and in the liver.

From the standpoint of food safety, that’s not good, said Dr. Jean Weese, an Alabama 
Cooperative Extension System food scientist and Auburn University associate professor of 
nutrition and food science.  It means food handlers who don’t wash their hands regularly or 
adequately can spread it to others, even though they may not experience the first symptom of 
listeriosis, the condition associated with exposure to the pathogen.  This raises the frightening 
specter of Typhoid Mary, the hapless carrier of typhoid fever who left scores of people sick and 
dead in her wake in the late 19th and early 20th centuries.

It’s especially bad news for immuno-compromised people --- AIDS and diabetes sufferers, the 
very young and the very old.  

While the most recent findings have instilled them with an even better understanding of the 
pathogen’s resilience, scientists have long held a grudging appreciation for listeria’s dogged 
persistence. 

“What really makes listeria such a serious pathogen is that, unlike other common pathogens such 
as salmonella and E.coli O157:H7, it can be airborne,” Weese said.

“The bacteria don’t have to be spread entirely through touching. It can be anywhere in this room 
--- not only in the air but on the floor and even on people,” she said, drawing an arc with her index 
finger.

Among foods, it is most commonly found in unpasteurized milk, soft-ripened cheeses and ready-
to-eat meats, such as hot dogs and pate. That explains why there has been such a heightened 
awareness about lunch meat safety within the last few years.  Other sources of listeria include 
raw and cold-smoked fish, raw meats and poultry, cooked poultry, fresh vegetables and ice 
cream.  

“Because these meats are exposed to air during processing, there’s the chance that some of 
these pathogens can attach to them.   And listeria can continue growing even after virtually all of 
the air has been vacuumed out in the course of packaging.”

Once the meat is opened and placed in the refrigerator, the bacteria multiply even more, she 
said.

“It’s almost like the proverbial rabbit in the briar patch,” Weese said.  “You’ve give it two of the 
conditions in which it thrives --- low-oxygen environments and colder temperatures.”

The detection of listeria in a meat product at an Atlanta supermarket recently prompted Vincent 
Giordano Corp. of Philadelphia to recall about 52,000 pounds of beef. Georgia health officials 
detected the pathogen on a deli meat product in a Kroger delicatessen counter.

Immune compromised people need to be especially careful, Weese said.

“If you have someone who is susceptible and they’re eating lunch meats, you need to microwave 
them before serving,” she said. “Just heating the food in the microwave at very low temperatures 
will kill listeria.”

Cheese included in the lunch packet should be prepared the same way, she said.

“Yes, I know among some people think this defeats the whole purpose of luncheon meats since 
they are supposed to be cold, but if you’re dealing with an immuno-compromised person, you 
can’t take any risks.”  

[Source:  Dr. Jean Weese, Alabama Cooperative Extension System Food Scientist and Auburn 
University Associate Professor of Nutrition and Food Science, 334-844-3269: Jim Langcuster, 
News and Public Affairs Specialist, 334-844-5686.]