Causing Major Recalls, Listeria is a Challenging Pathogen

Recent massive recalls due to contamination by Listeria monocytogenes have raised new concerns about gaps in surveillance and processing that could lead to outbreaks. Blue Bell Creameries of Brenham, Texas, issued recent recalls related to a Listeria outbreak in its frozen products. Dallas, Texas-based Sabra issued a nationwide recall of hummus after random testing identified Listeria in a sample of hummus at a Michigan retail store. Some other significant outbreaks of Listeria in recent years included Granny Smith and Gala apples recalled by Bidart Bros., of Bakersfield, Calif., and cantaloupes produced on a farm in Colorado.

Listeria can cause serious illness in people with compromised immune systems and in unborn babies. Among those who develop listeriosis, mortality is about 30 percent. Due to the high risk of the illness, the government has established a zero tolerance policy for Listeria contamination, meaning that there can be no detectable bacteria in a 25 gram sample of food.

The range of variety in types of food production facility demonstrates the opportunism of the Listeria organism. That flexibility, as well as an ability to grow in refrigeration and a resistance to ordinary cleaning methods makes Listeria a difficult pathogen to entirely eliminate and control, according to Lynne McLandsborough, PhD, associate professor of food science at the University of Massachusetts Amherst.

“One thing Listeria can do, if there are nutrients and moisture, is it can grow in a biofilm,” Dr. McLandsborough says. When it forms a biofilm, the bacteria produces an exopolymeric substance that can make it resistant to ordinary cleaning products and methods like chlorine bleach.

In reviewing the Blue Bell inspection reports, Dr. McLandsborough says that it was noted that there was a lot of condensation in the production facility. One possible route of contamination, in the presence of condensation, is that water could drop intermittently onto a food-processing surface. That would lead to inconsistent contamination that could easily be missed in product testing.

Another challenge with Listeria is that it can have a lengthy incubation period—up to four weeks, according to Dr. McLandsborough. That makes it difficult to track illnesses to a source.

To prevent Listeria contamination, Dr. McLandsborough recommends taking no chances with cleaning of the production facility. “Contact a cleaning service,” she says. There are a number of sanitation companies that specialize in cleaning food production facilities and offer an appropriate suite of products to remove Listeria biofilms.

Blue Bell signed agreements with health officials in Texas and Oklahoma on May 14 requiring the company to inform the states whenever there is a positive test result for Listeria in its products or ingredients.