I’m not sure you can say that Listeria monocytogenes is pathogen enemy number one, but it’s definitely on the most unwanted list. It can be virtually everywhere, and there is growing public awareness of what it can do when left unchecked. The recent news about recalls only reinforces what many of us already know—an outbreak is possible, traceable, and has very real consequences for both business and consumers.
Listeria monocytogenes, commonly referred to as Listeria, is a pathogen that causes listeriosis, a serious human illness that is fatal in about 20 percent of cases. Unlike most other foodborne pathogens, it can grow at proper refrigeration temperatures. In addition, Listeria is widely distributed in nature; the organism has been recovered from farm fields, vegetables, animals, and other environments such as food processing facilities, retail stores, home kitchens, and ready-to-eat foods.
As the graphic shows, understanding the ecology of Listeria is the first step to its management and ultimately, to protect public health. This understanding can help identify the optimal control measures, effectively aimed at the most likely sources of the organism to keep them in check.
It’s incumbent that any controls be validated to demonstrate their effectiveness, then implemented consistently and routinely verified to ensure that they are carried out as expected. As new information becomes available about Listeria and its possible sources, these controls may need refining.
Considerations for Management
So what can you do to effectively control Listeria? The short answer: remain vigilant and focused on continual improvement. Here are some considerations for effective management of Listeria.
1. Know your enemy: understand the ecology of Listeria. Keep in mind that this information must be refined as new facts emerge. You can’t effectively manage a pathogen without a complete understanding of what it is and the environment in which it thrives. Listeria has been isolated from a wide variety of raw agricultural products including raw meats, poultry, seafood, and milk. It is found in soil and in silage and persists in nature and in processing environments in niches where it can evade control mechanisms. It can also sometimes be enmeshed in biofilms where the cells may be protected from the effects of sanitizers.
Parameters for growth of the organism allow it to thrive under conditions that are more extreme than what other pathogens may be able to stand. According to International Commission on Microbiological Specifications for Food, Listeria cells are able to grow over a temperature range of 32 degrees Fahrenheit to 113 degrees Fahrenheit, a pH range of 4.4 to 9.4, and aw limits from 0.92 to 0.992. This means that Listeria may be a hazard in foods containing higher levels of salt or sugar and in refrigerated items.
Listeria is a persistent organism. R.B. Tompkin reported survival in dairy facilities as long as seven years; in a fish processing plant, four years; and in a poultry facility, up to 12 years. Further evaluation of the literature reveals that the concept of persistence is complex and requires greater scrutiny of the data in the context of the actual production environment. What has emerged is that a clearer understanding of the food environment is needed for optimal control. Because Listeria is abundant in nature and can be found almost anywhere, there can be a constant reintroduction of the organism into the food plant, retail setting, food service establishment, and home. It is difficult to totally eliminate this contaminant from the food-handling environment, but the goal is to control it as effectively as possible, especially where it can contaminate ready-to-eat, refrigerated foods.
2. Identify controls to manage Listeria sources. The greater the ability of a food to support growth of Listeria, the greater the risk. The relatively small proportions of foods that may be contaminated with high levels of Listeria monocytogenes pose the greatest risk. Contributing factors to overall listeriosis risk include consideration of the:
- Ability of the food to support the growth of monocytogenes;
- Amount and frequency of consumption of a food;
- Frequency and extent of contamination of a food with monocytogenes;
- Temperature of refrigerated/chilled food storage; and
- Duration of refrigerated/chilled storage.
Key to effective risk management is consistent application of Listeria control measures.