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.
One measure is prevention of growth through time/temperature control or formulation control. Intrinsic characteristics of a food (e.g., pH, aw) can be leveraged in the product development process to build in effective growth controls. Prevention of growth is essential; Listeria can grow at refrigerated temperatures, which defeats one of the traditional food safety measures. However, the organism is killed by normal food pasteurization and cooking processes, and is typically sensitive to most sanitizers when used at recommended rates and as long as it is not shielded within biofilms. Still, contamination may occur after the cooking process in the processing environment, at retail locations, and in the home. For example, post-pasteurization contamination of food products can occur when the organism is dispersed via an aerosol.
The other essential part of effective control is prevention of contamination of the food through all aspects of the food handling process. This starts with preventing entry of the organism by controlling incoming contamination that can originate with employees, equipment, ingredients, and packaging. Movement of people, equipment, materials, etc., must be monitored, controlled, and restricted as appropriate. Preventing growth in the food handling areas relies on the removal of growth nutrients, including water and soil. Keep areas as dry as possible, keep temperatures low, and have sound sanitation practices. Appropriate sanitation programs include the following considerations.
- Match the cleaner to the nature of the soil;
- Match the cleaner to the water properties;
- Optimize solution compatibility with the surface;
- Ensure the cleaner is appropriate for the method of application;
- Use products that meet environmental guidelines;
- Follow sanitation standard operating practices; and
- Seek guidance from sanitation providers.
- Use EPA-registered products that have a claim against Listeria monocytogenes on the label; and
- Follow the manufacturer’s labeled instructions.
In addition, knowledge of potential harborage sites is important, as contamination is more likely to occur when the organism has become established in a niche where it may be able to evade control measures. Good sanitary equipment design and proper maintenance, in combination with regular, effective, and thorough sanitation, can help eliminate Listeria from niches. Targeting sanitation to the areas where Listeria can be harbored is essential. Food processing plant surveys have found Listeria in the following locations: floors, drains, coolers, cleaning tools, product and/or equipment wash areas, food contact surfaces, condensate, walls and ceilings, and compressed air.
Since it’s essential to detect and manage harborage sites with thorough and frequent sanitizing to control Listeria, a program should include daily sanitation of floors and drains and adequate attention to less frequently cleaned areas such as HVAC systems, walls, coolers, and freezers. Also, damaged equipment, cracks, crevices, and hollow areas must be part of sanitation and inspection schedules. It is essential to avoid creation of aerosols during cleaning, especially of floors and drains, to avoid spread of contamination.
Finally, controlling Listeria also means considering the roles of various transmission routes. The same vectors that can bring Listeria into a facility also need to be controlled once they are inside. These include employees, forklifts, cleaning tools, pests, water, air, etc.
3. Validate the effectiveness of controls. Any control measure implemented with the intent of managing Listeria needs to be chosen carefully and shown to be effective. This is the crux of validation and involves the act of collecting and evaluating scientific and technical information to determine that the control measure, when properly implemented, will achieve the intended result.
For Listeria control, this can come from a variety of sources such as scientific support, including published studies or references; advice from experts including food safety personnel, academics, consultants; and/or in-plant expertise gained from extensive experience. These resources can help assess whether the considered control is theoretically sound. But, the validation process needs to go a step beyond these scientific principles or advice, and must extend to studies demonstrating that the control can be implemented at the site per the plan to achieve the intended results. Evidence must show that the control can be effectively implemented as designed.
This latter part of validation gets at the practical aspects of a control program, answering this critical question: how capable are the facilities at impeccably implementing the controls? In this part of validation, one needs to determine how to apply scientifically sound information to the particular process and plant. Possible considerations might include the availability of personnel to carry out the control, their level of expertise, the tools that might be needed, the type of equipment available, the age and condition of the facility, etc. This requires in-plant data collection, test results and other information demonstrating that the control can be operated within the particular establishment. The USDA Food Safety Inspection Service offers practical guidance on validation.
4. Implement the controls. Once controls are identified and determined to be valid, they need to be continuously practiced, regardless of the operational or facility-specific factors that might affect them. Meaning, sales are up and you want to make more of a particular food product? Great, but the controls and practices cannot change. There needs to be continuous attention to these control mechanisms since they are directed against a serious potential hazard. This is the crux of establishment and practice of a food safety culture. A rule of thumb is to ensure that there is support to continually practice the control measure “irrespective of who’s looking!”
Leadership is essential. Senior management must provide appropriate resources including personnel, supplies, materials, etc. to carry out the controls. In addition, they need to offer support to the various team leaders and members to emphasize the importance of strict attention. Management must be engaged and keep employees well informed of the potential extent of the Listeria hazard and underscore the importance of the team’s work to actively manage the risk.
5. Verify that controls are consistently applied and be ready to modify them if needed. A key part of implementation is to verify that the system is operating according to the pre-determined plan. This is verification and this practice must be applied to every specific control measure to ensure that each is applied according to design. It allows for ongoing assurance by the plant that the control was done as designed.
There are many ways to verify. The particular facility needs to determine which procedure is best for its operation. Fresh eyes can help, so a review of records by someone other than the individual who recorded the results is a common practice. Another way is thermometer or water activity meter calibration or ensuring that plant equipment used for cooking is adequately set up. Sanitation verification that the surface is free of sensory detectable soils may be done via inspection for visual cleanliness or by sight, smell, or feel. Other sanitation verification practices include swabbing for residual adenosine triphosphate (ATP) after the cleaning step to show that the surface is free of detectable animal, plant, microbial, or human ATP.
Microbiological swabbing is also commonly used to verify effectiveness of sanitation. It should be done just after the sanitizing step or immediately before manufacturing if there is an extended time during which the sanitized equipment sits before production startup. Microbial swabbing could also be done at other times to identify Listeria harborages and allow for prompt implementation of appropriate corrective actions. Recently published guidance documents from Grocery Manufacturers Association and United Fresh Produce Association provide excellent information to help direct the development of a comprehensive Listeria swabbing program and provide input on the necessary corrective actions that could be followed. Although L. monocytogenes is the only member of the Listeria family that causes human illness, the presence of Listeria species in a food processing or handling environment may indicate that conditions are favorable for L. monocytogenes and appropriate actions should be taken.
Over time, things change in a processing plant and Listeria controls need to be validated regularly to ensure they remain effective. Data collected from regular verification should be input to this process. Consider any significant changes that may impact these controls and prompt re-validation of the controls. This typically includes changes in formulation, production processes, equipment, scientific information, staffing, and water (source or season).
Effective management of Listeria requires awareness of the particular characteristics of the organism and identification combined with ongoing diligent practice of valid controls to address its potential risks. Such actions must be verified and constantly assessed to ensure that they are continuing to make a difference.
The process of identifying and implementing the optimal measures needs to be reinitiated if there is sufficient evidence through swabbing or other data such as new information obtained from outbreaks, published research, or regulatory commentary that indicates a possible loss of control. Listeria control requires unwavering and strict attention to many factors to ensure that it remains effectively managed. Not to be discouraged, it’s challenging but essential.
As they say, the proof is in the pudding. Or in this case, what’s NOT in the pudding.
Dr. Petran is vice president, food safety and public health, at Ecolab. Reach her at Ruth.Petran@Ecolab.com.