The current outbreak of listeriosis linked to recalled Queseria Bendita soft cheeses is a reminder that these types of foods have often been recalled for Listeria monocytogenes contamination in the past. In the current outbreak, three people in Washington state have been sickened and one person has died.
Over the past three years, there have been at least 25 recalls of soft cheese for Listeria monocytogenes in the U.S. and 9 recalls in Canada for the same reason. And there have been recalls of hard cheeses for Listeria too.
Cheese is particularly vulnerable to Listeria contamination for a few reasons. Any cheese, whether hard or soft, made with raw or unpasteurized milk has a much higher risk of contamination, since Listeria bacteria are present in the farm environment. In fact, Listeria is one cause of mastitis in cows. Listeria is found in soil, people, animals, plant residue, packing sheds, and processing systems.
Processing conditions also support Listeria growth, especially in soft cheeses. While they ripen, they are sometimes coated with a brine or a smear. This substance can be contaminated with Listeria bacteria. The cheeses are sometimes turned by hand; this can also introduce bacteria. And the moist environment used for making and ripening cheese is ideal for bacterial growth.
Soft cheeses have a higher pH, or lower acidity, and lower salt content than hard cheeses. These conditions promote Listeria growth as well. And Listeria monocytogenes grows at refrigerator and freezer temperatures, the traditional methods for controlling bacteria.
The bacteria is very hardy. It can persist in a food facility or retail store despite thorough cleaning and sanitizing. Even super cleaning doesn’t always kill the bacteria.
Once Listeria bacteria is introduced into a facility it can be very difficult to eradicate. In fact, a 2004 study by Cornell University found that the bacteria can persist in a processing environment for a year or even longer. When cheeses are cut and packaged, Listeria can be introduced if workers don’t wash their hands or are ill. If a worker puts equipment on the floor to clean, it can become contaminated.
All of these facts mean that vulnerable populations should avoid consuming soft cheeses, as well as other foods that are susceptible to bacterial contamination such as raw milk and cider, smoked meats, deli meats, and undercooked and raw meats, poultry, and eggs. Pregnant women, young children, the elderly, and people with compromised immune systems and chronic illnesses are especially vulnerable to Listeria monocytogenes food poisoning.
