Co-authored by Lily Yang, Ph.D. and Rolando Gonzalez, Ph.D., The Acheson Group
Introduction to Listeria
Members of the bacterial genus Listeria are frequently found in the environment, soil, and vegetation; however, the species Listeria monocytogenes (L. mono) is a significant pathogen of concern in the food industry and is a hardy organism capable of thriving in conditions beyond what other organisms can survive in. 1 L. mono can flourish in various environments or food products, including high-salt environments, moist conditions, and more terrifyingly, at refrigerated temperatures - even below 33.8˚F!
L. mono’s hardiness and ability to survive in cold temperatures and amongst different foods make this pathogen especially insidious. Many foods have been associated with outbreaks, including milk (especially raw or improperly pasteurized), soft cheeses, ice cream, raw vegetables, raw or smoked meats/poultry/fish and seafood (including sushi products), and deli meats (including fermented raw-meat sausages, hot dogs, etc.).
Due to Listeria’s ubiquity and hardiness, it is imperative that food retailers and operators not only understand the conditions in which Listeria can persist, but also ensure that conditions are kept which help to minimize its growth. These best practices include ensuring proper hot/cold temperatures for holding food are maintained, and cleaning and sanitizing refrigerators, display cases, and frequently used kitchen equipment, while also maintaining the sanitary conditions of one’s establishment. Listeria can proliferate in harborage points, including crevices and hard-to-clean places! L. mono also has the uncanny ability to form biofilms, which allows it to grow layers upon itself and create an even more durable and hardy structure which can make it difficult to clean, sanitize, and ultimately eradicate.
L. mono outbreaks have been associated with various food settings. More commonly, they have been identified in ready-to-eat (RTE) products available at retail stores and deli counters, including a 2021 outbreak related to queso fresco (a soft cheese). In 2020, L. mono outbreaks associated with deli meat (including salami, mortadella, and prosciutto) and another associated with hard-boiled eggs occurred. While the number of sickened individuals was low, most were hospitalized, and at least one death occurred in each outbreak. Listeria can be found in various nooks and crannies throughout a facility, including those of hard-to-reach parts, blades, cart wheels, and even grease catches and drains. Lack of frequent sanitation of these locations can increase the risk of L. mono cross-contaminating food contact surfaces in these settings.
Listeriosis – A Serious Illness Caused by Listeria monocytogenes
The CDC estimates that 1,600 people fall ill, and 260 individuals die each year in the United States due to Listeriosis. 2 L. mono’s mortality rate is between 15-30% in all cases, and infections primarily affect pregnant women, newborns, adults over the age of 65 years old, and individuals with lowered immune systems.
Symptoms of listeriosis vary widely and range from fever, muscle aches, nausea, and gastrointestinal distress to, in more serious cases, a nervous system infection resulting in headaches, stiff necks, confusion, loss of balance, convulsions, and even death. According to both the FDA and CDC, the onset of listeriosis can occur anywhere between a few hours to a few days, or even up to three months, and can last for a few days to several weeks. Can you imagine finding out that your symptoms and illness could have been caused by something you’d consumed months prior? In healthy individuals, illness from Listeria may not result in any symptoms, or these may be mild, while in younger, older, and immune-compromised individuals, illness can result in death.
In retail and foodservice establishments, prevention of L. mono ultimately falls into the hands of the employees and staff. Not only is it imperative that you are mindful of your food safety program, those of your suppliers, and any food recalls that may be affected by Listeria, but you must also be aware of the conditions and environment of your facilities. Due to the combined factors of Listeria being able to survive in various conditions and form biofilms, retail and foodservice establishments must ensure they are maintaining proper hygiene and food safety standards while minimizing the potential for cross-contamination. Thus, food safety awareness is necessary for all staff members with the reminder that ultimately, their actions (or inactions) can impact the end consumer.
How to Control Listeria monocytogenes in Food Handling Settings
Listeria prevention starts by eliminating sources of L. mono from your establishment. Common sources of L. mono include food products and raw ingredients, environmental niches, unsanitary equipment, and personnel (employees, customers, and vendors). By properly managing these sources, including incorporating effective sanitizing solutions and practices, the risk of your establishment becoming the source of an outbreak can be minimized.
Here are some tips to minimize the risk of L. mono in your establishment:
- At all times, maintain all display, storage, and refrigerator or cooler cases to an internal temperature of 41˚F or lower while ensuring adequate airflow. Temperatures should be set even lower during busier periods when cases may be opened frequently to minimize the loss of cooler temperatures. Additionally, food products should NOT be stacked so that airflow is impeded, as this interferes with the ability of the equipment to function properly.
- Evaluate the sanitary design of facilities, equipment, and tools to ensure they are easily cleanable and harborage points will not be missed. Follow manufacturers’ guidelines for equipment, tools, and utensil use. Ensure all facilities, equipment, and tools (including utensils) are designed to be easily cleanable, having safe, smooth, nonporous, and durable surfaces. At a minimum, equipment, tools, and utensils should be certified by the American National Standards Institute (ANSI). Not only are such surfaces easier to clean, but they will also be less likely to have cracks, pits, or chipping (which, while unseeable by the naked human eye, are easily harborage points by which L. mono and its biofilms can flourish). Equipment should be installed in areas that can also be easily cleaned, sanitized, and away from contamination. Routinely inspect equipment, utensils, and tools for chips, cracks, and missing parts (including screws, bolts, gaskets, etc.); if any incursion occurs, it should be quickly dealt with and promptly replaced. Especially in a deli-counter production facility, knowing how your equipment can be broken down and reassembled can also provide insight into the cleaning and sanitizing process to ensure all surfaces are appropriately addressed.
- Properly clean and sanitize equipment and environmental surfaces to prevent L. mono harborage or buildup. It is imperative to implement ongoing and daily sanitation procedures. In areas where there are increased food preparation activities, you may consider more frequent and thorough cleaning procedures. It is essential to identify and use cleaning and sanitizing solutions formulated for the proper equipment or environmental surface and that the equipment be broken down as much as possible for cleaning. On equipment, frequent L. mono harborage points include slicers, prep tables (especially when there are cracks or pits or missing bolts), cart wheels, cooling fans in display cases, and the actual display cases and coolers. Ensure that tools used for cleaning are cleaned and sanitized after each use and remain in good working condition. Other L. mono harborage sites within facilities can include grease traps, floors, walls, air vents, and anywhere with pest activity.
- Clean and sanitize floors frequently and minimize standing water. As discussed, Listeria persists in wet environments. Not only should floors be cleaned and sanitized daily, but it is also essential to focus on floor drains. Because floor drains have cracks and grates that can be inset, they too can become harborage points – especially as water and debris can pool into gaps.
- Manage product flow to minimize and prevent cross-contamination. Additionally, manage, monitor, and limit the shelf-life of RTE products that have been opened, prepared, and/or are being held at temperature to no more than 24 hours. While heat and most processing methods (and chemical treatments) can destroy L. mono, food products can be contaminated post-processing by both people and the environment. Therefore, it is crucial that if an RTE product has been taken out of the refrigerator, it should be promptly put back to slow pathogen growth. In delis, it is best to minimize pre-slicing ingredients until the customer requests it. Just as importantly, minimize maintenance or construction activities in areas where food is being prepared as these actions can increase the potential for contamination. Finally, keep food activity functions within their own space while segregating other activities from that space to minimize cross-contamination.
- Ensure employees are utilizing proper hand hygiene and gloving to eliminate bare-handed contact with food. Ensure that employees are wearing and changing gloves, as necessary, to decrease cross-contamination between RTE-specific foods. Additionally, proper hand hygiene (including how and when to wash hands, as recommended by the U.S. Food Code) can reduce the incidence of transferring L. mono amongst foods products and even within your establishment.
- Have a supplier verification program and manage incoming ingredients with approved suppliers and approved sources. Seek to purchase solely from approved sources with food safety programs in place. Additionally, you should ensure that food safety is always part of your supplier specifications and requirements, and work with your suppliers to understand their pathogen prevention and environmental programs.
- Enact continuous training opportunities with employees to reinforce food safety practices. As part of training, you should ensure that employees are also cognizant of “what to do” if a piece of equipment, tool, or even the facility must undergo replacement or repair (so that they too may decrease the potentials for contamination).
While L. mono is a dangerous pathogen frequently encountered in food handling settings, it can be controlled by focusing on building a best-in-class food safety plan. This plan should ensure that conditions which foster Listeria growth in the food environment are minimized, and that sanitation procedures are executed properly and frequently to minimize any sporadic contamination that may occur within the facility. Following these best practices will help to minimize risk of Listeria contamination in your establishment.
Download our new PURELL® Food Safety – Pathogens of Concern: Actionable Research Bulletin, “Listeria monocytogenes – What You Should Know About This Rare but Serious Bacterial Foodborne Pathogen,”