Listeria Updates

Recent articles published in Food Safety News (*) provide important information about Listeria monocytogenes (Listeria), how it causes foodborne illness and the importance of public awareness about the illness. These articles implicate smoked salmon (4/26/24), smoked fish in Lithuania (12/14/23) and enchiladas (4/22/24) to name a few cases.

Looking at the reported cases of illness, it is easy not to take this illness seriously. That, however, is a tragic error. Listeria causes two types of foodborne illness. While most cases are non-invasive and relatively mild, the invasive illness (listeriosis), associated with highly susceptible populations (young, elderly, pregnant women, immune-compromised) account for 19% of deaths and nearly always require hospitalization. Nearly one-quarter of pregnancy-associated cases result in fetal loss or death of the newborn. The Centers for Disease Control states that, while symptoms usually start within 2 weeks after eating food contaminated with Listeria, they may start as early as the same day or as late as 10 weeks after. This makes it difficult to trace the foods involved. It may create a false sense of security.

Concerns about listeriosis led to research about the public’s awareness of the illness. Studies in the Netherlands and Iceland showed that high risk populations were either unaware or unconcerned about the problem(*). Younger people followed food safety guidelines more strictly than the elderly. The following guidelines were included as critical in public information campaigns in these two countries:

1. Consumers should maintain refrigeration temperatures below 39.2 degrees F and use ready to eat foods within seven days.

2. Analysis of measured temperatures of 534 refrigerators on the bottom shelf showed they varied from 30.2 to 62.6 degrees F, with two-thirds showing 42.8 degrees F or lower, according to the study published in the International Journal of Food Microbiology(**)

Note: At first, this seems hard to believe. However, even a commercial food service refrigerator can have a wide range of air temperatures from the front to rear of the unit. Overcrowding and covered shelves easily create warm spots and warm food. A good plan is to place thermometers in the front and rear of a large refrigerator and not to rely entirely upon the inset thermometer at the front..

3. Careful cleaning and maintenance are key to preventing problems with listeria. The Iceland public information campaign advised cleaning food contact surfaces properly and frequently to avoid the development of biofilms, a favorite ‘hiding place’ for listeria. Maintenance was also emphasized to stop the bacteria from surviving or multiplying in the production environment

4. Finally, researchers wanted to know the potential sources of cross contamination and which factors contribute to the survival of Listeria monocytogenes in such environments. Floors in different nonfood contact areas were sampled: a preparation area, where ingredients were kept at 39.2 degrees F, and a production area where food was assembled and packaged, kept at 50 degrees F. The two sites were sampled over 10 weeks, before and after cleaning.

Sample results showed a stable community of both Listeria and other bacteria which coexist together, support each other and adapt to sanitation introduced by food safety controls (listeria, for instance, once partially exposed to adverse conditions, has a shorter and faster growth rate) The composition of these groups changes throughout the facility, based on air temperature and use of the area. Bacteria in this facility were established populations and, while sanitation might reduce its numbers, the composition did not change by the movement of employees between areas

Any cleaning program, therefore, must address the entire community, in order to control Listeria. Controlling cross contamination from the environment, employees or incoming deliveries remain critical factors to reduce the numbers of bacteria: preventive controls, however must provide a complete elimination of Listeria, However, a final point is provided by a chief researcher, a point to emphasize the resilience and ability of Listeria to change and adapt….

“As Listeria monocytogenes is supported by a stable community of other bacteria, we may now need to develop new strategies to alter the whole bacterial population to effectively eliminate the pathogen”

(*) Food Safety News 4/25/24 “Listeria Cases Prompt Warning in Iceland” and “Dutch Study Looks at Fridg Temperatures and Listeria Risk” a
(**) Wieke P. van der Vossen-Wijmenga, “Temperature status of domestic refrigerators and its effect on the risk of listeriosis from ready-to-eat (RTE) cooked meat products” International Journal of Food Microbiology, V. 413 (2) March 2, 2024

(***)“Scientists study Listeria survival in food factories” Food Safety News 4/5/24