Listeria monocytogenes is estimated to cause nearly 1,600 illnesses each year in the United States and is responsible for more than 1,400 hospitalizations and 250 deaths.[1] Nearly all cases of Listeria infections (listeriosis) result from persons eating food contaminated with L. monocytogenes or affect newborn infants whose mothers eat contaminated food during pregnancy. Rates of listeriosis per 100,000 individuals are relatively consistent across nations that report; estimates for cases of the febrile gastrointestinal form of the disease are rarely considered, even though a few outbreaks have been reported.[2] Outbreaks from L. monocytogenes are not frequent compared with those caused by pathogens like Salmonella and Clostridium perfringens, but they often generate national or even international notoriety. This is because: 1) the resultant infections are serious, with hospitalizations and deaths; 2) they are often caused by lack of management oversight in manufacturing plants, lack of understanding of the nature of the pathogen or where errors by workers have been allowed to repeatedly occur; 3) these plants are then found to be typically out of compliance with existing regulations; and 4) the outbreaks have major economic consequences, especially if the products affect international trade. Outbreaks have involved a whole range of facilities, from small artisanal operations to well-established major processors. The implicated foods are typically deli meats and artisanal cheeses. However, case-control studies in the U.S. have implicated melons and hummus, and in 2011, a large outbreak associated with contaminated cantaloupes causing 147 cases and 33 deaths reinforced a risk of listeriosis from consuming produce items. Most listeriosis outbreaks have been reported from Europe, the U.S., Canada, Australia and New Zealand, but the pathogen is likely causing illnesses in many other nations that do not investigate cases, which are mainly sporadic. Even in the U.S., where listeriosis is nationally notifiable, it is believed that many cases are not recognized and reported. The fact that contaminated ready-to-eat (RTE) food is responsible for almost all illnesses—and the organism surprises us in the range of food it can contaminate in numbers large enough to cause outbreaks—has forced both government and the food industry to make a major effort to reduce the public’s exposure to the pathogen.

Characteristics of Listeria monocytogenes
Among pathogens, L. monocytogenes has distinctive characteristics that must be recognized before implementing prevention and control strategies applied to the food processing and food preparation environments. These include the extensive occurrence of the pathogen in the natural environment. Listeria spp., including L. monocytogenes, are present in soil, water, sewage, vegetation and wild animal feces, as well as on the farm and in food processing facilities.[3] Farm animals can be asymptomatic or cause encephalitis, septicemia and abortions, and may be a source of L. monocytogenes on the farm. The pathogen can survive and grow at a wide range of temperatures (-1 to 45 °C); for instance, it can be a long-term resident in chilled food processing premises under high humidity, but it can also survive conditions of low water activity, such as under high salt concentrations.[4] Some strains can survive for long times under adverse environmental conditions and persist in niches in food processing equipment and associated drains, walls and ceilings; the main reason for this is L. monocytogenes’ ability to form biofilms on many surfaces.[5] It is also considered a highly pathogenic facultative intracellular organism, which actively invades hosts and induces listeriosis; infection is normally associated with ingestion of large numbers of the bacteria in healthy adults, but likely much smaller numbers can affect immunocompromised individuals. Many of these factors are unique and differ from those of other foodborne pathogens such as Escherichia coli, Salmonella and C. perfringens. Generic pathogen reduction programs may be insufficient to take into account the pathogen’s ability to grow at low temperatures and its persistence within biofilms. Another issue is that there may be a continual source of the pathogen via raw material entering a processing facility, carrying with it both transient and persistent strains. Some strains have been shown to persist for months or even years in such environments.[6]

    All countries should consider making listeriosis a notifiable disease and have in place both active and passive surveillance systems for noninvasive gastrointestinal infections. In 2005, the U.S. Centers for Disease Control and Prevention launched its Listeria Initiative for state and local health departments to follow up on all listeriosis patients as soon as their cases are reported and to collect relevant data using an extended questionnaire. This enhanced surveillance system contributed substantially to the relatively quick resolution of a Listeria outbreak linked to cantaloupes in the summer of 2011. One of the big questions is why there is an increased proportion of elderly people suffering from listeriosis in developed countries. A recent survey showed an increase in risk of listeriosis among older persons, pregnant women and Hispanics in the U.S.[7] Comparative analyses of strains worldwide are essential to the identification of novel outbreak strains and epidemic clones. For instance, sequence typing confirmed that a predominant L. monocytogenes clone caused human listeriosis cases and outbreaks in Canada from 1988 to 2010.[8]

Food Vehicles
Even though most attention has been directed to controlling contamination of meat and poultry products, a variety of products have recently been implicated as vehicles of L. monocytogenes. These include several RTE foods, such as hard-boiled eggs, sandwiches and hummus, as well as lettuce, celery, cabbage and walnuts. Produce is clearly now a major vehicle for risks of outbreaks of listeriosis, and these have been linked to poor storage conditions and environmental cross-contamination after processing. However, different types of the implicated produce seem to have different growth rates and maximum L. monocytogenes population densities on subsequent experimentation. This may depend partly on the surface structure of each vegetable or fruit and the way it is harvested. For instance, lettuce and spinach leaves are fragile and can leak contents, and cantaloupes’ rough surface allows bacterial harborage. A number of predictive models for the growth kinetics on different items have been carried out on different products such as white cabbage, lettuce and cantaloupe.

Much attention has been placed on reducing risks in deli meat production facilities because of the many outbreaks that have been linked to these foods over many years; the last major outbreak was in Canada in 2008 with 58 confirmed cases and 22 deaths. The U.S. Department of Agriculture (USDA) conducted a risk assessment for deli meats but focused on risk management questions.[9] They found that 1) food contact surfaces (FCS) positive for Listeria spp. greatly increased the likelihood of finding RTE product lots positive for L. monocytogenes; 2) the frequency of contamination of FCS with Listeria species encompasses a broad time frame, and the duration of a contamination event lasts approximately a week; 3) minimal testing was estimated to result in a small reduction in the levels of L. monocytogenes on deli meats at retail, and an increased frequency of FCS testing and sanitation was estimated to lead to a proportionally lower risk of listeriosis; and 4) combinations of interventions (e.g., testing and sanitation of FCS, pre- and postpackaging interventions and the use of growth inhibitors/product reformulation) appeared to be much more effective than any single intervention in mitigating the potential contamination of RTE product with L. monocytogenes and reducing the subsequent risk of illness or death. A subsequent USDA Food Safety Inspection Service study showed that deli meats prepared at retail had a far greater risk of causing listeriosis than prepackaged deli meats.[10]

Many cheeses made from unpasteurized or even pasteurized milk allow growth of L. monocytogenes, and surveys of artisanal cheeses in different countries have shown them to be contaminated. For instance, the pathogen can grow in Mexican-style queso fresco at both 10 °C and 4 °C to reach a maximum population density greater than 7 logs.[11] The use of antimicrobials and/or postprocessing interventions is recommended to prevent the growth of the organism. However, in some artisanal cheeses, the lactic acid bacteria are natural inhibitors. Nevertheless, the 2012 U.S. Food and Drug Administration/Health Canada draft risk assessment found that the risk of listeriosis from soft-ripened cheeses made with raw milk is estimated to be 50 to 160 times higher than that from soft-ripened cheese made with pasteurized milk.[12]

L. monocytogenes is persistent in fish and shellfish processing plants. Cross-contamination can originate in external and internal sources. In British Columbia, more fish facilities than dairy and meat facilities had FCS and RTE fish foods contaminated with Listeria spp., and increased inspection is recommended.[13] In Scandinavia, there is a high prevalence of contaminated cold-smoked and gravid fish that have caused outbreaks.[14] Sweden plans to halve the prevalence of L. monocytogenes in these products by the end of 2015. Research has shown that a combination of any two of nisin, lysozyme and e-polylysine antimicrobials effectively inhibit growth of L. monocytogenes in RTE seafood. However, some traditional processes may also discourage the survival of L. monocytogenes. For instance, the pathogen decreases during storage in vinegar-marinated sushi rice with raw salmon and halibut.[15] Although electrolyzed oxidizing (EO) water has not been shown to be effective against L. monocytogenes on fish surfaces directly, removal of fish residue from processing equipment such as conveyor belts and slicing machines and their exposure to EO water could assist in reducing biofilm formation.[16]

In processing plants, both FCS and non-FCS can be important reservoirs for Listeria spp. In fact, a false sense of security can occur when only FCS are tested, since the main reservoirs can be as varied as wet floors, drains, shoe soles, equipment brackets and stair treads. In the 2011 cantaloupe outbreak, there was condensation from cooling systems draining directly onto the floor; poor drainage, resulting in water pooling around the food processing equipment; difficult-to-clean food processing equipment; and no antimicrobial solution in the water used to wash the cantaloupes. Sanitary equipment and plant design are crucial for controlling contamination. Because of a study of a cooked frozen chicken meat operation that showed Listeria spp. was most frequently recovered from the liquid nitrogen chiller exhaust pipe, the metal detector conveyor belt and the freezer drain, the plant’s cleaning and sanitizing procedures were revised and strictly implemented to reduce and eliminate the sources of Listeria contamination.[17] In retail deli operations, hands and gloves have been identified as important potential contamination sources. Pathogen transfers are likely to occur from non-FCS (floor drains, walk-in cooler floors and knife racks) to FCS, and from FCS (cutting boards and preparation sinks) to product. According to a report by the 2004–2006 Conference for Food Protection L. monocytogenes Intervention Committee,[18] sanitation programs to specifically address L. monocytogenes consist of three actions: 1) effective removal of soil; 2) an effective rinse step and 3) proper application of a sanitizing agent, which includes contact time, concentration and temperature. A sanitation program should also implement written procedures for proper cleaning and sanitizing FCS and non-FCS. These procedures should include the frequency of cleaning, chemicals to use, instruction on how to perform the task and the steps to verify it is being done correctly. A visual examination of all FCS should be done before the start of operations to ensure compliance with cleaning procedures and to take corrective action if necessary. Written procedures for food establishments should include the cleaning and sanitizing of maintenance tools. Every food establishment must have a method for verifying the effectiveness of its cleaning and sanitation program. The effectiveness of sanitation programs can be verified in different ways, and often a combination of approaches can be used. When determining which method to use, consider factors such as:

•    How difficult the area is to clean
•    Whether possible L. monocytogenes harborage sites are present
•    Whether there have been previous problems with sanitation

The person in charge should be responsible for ensuring that employees are properly trained for the tasks assigned to them and that they fully understand how to perform the sanitation procedures. This includes mixing and testing cleaning and sanitation solutions for proper strength, cleaning and sanitizing certain equipment according to a prescribed schedule and checking to be sure equipment and surfaces are cleaned as needed throughout the day. Some of the methods that can be used to verify the effectiveness of sanitation programs include:

•    Observation and monitoring
•    Rapid sanitation tests
•    Microbiological testing

These methods vary by cost and level of technical expertise needed to use them.

The sanitation and overall control programs will be determined by whether the final product allows growth of L. monocytogenes. Because of the diversity of RTE foods being produced, the processes used and the prevention and control strategies, companies need to have challenge tests done on their RTE food products if there is uncertainty that L. monocytogenes may or may not grow during the shelf life of the product.[19]

One of the big concerns for processors is biofilm formation. Persistent strains may not be better than transient strains for biofilm formation, but they seem to resuscitate faster than non-persistent ones after treatment.[20] Biofilms are generally resistant to standard cleaning and disinfecting systems. One suggestion is to scour them off FCS with scallop shell powder.[21] However, most plants use sanitizers exclusively, and while there are many available for cleaning equipment, not all are equally effective against L. monocytogenes biofilms, and the results of experiments do not always agree with each other. In one study of 21 commercial sanitizers tested,[22] peroxyacetic acid (PAA), chlorine dioxide and acidified sodium chlorite-based products gave the best decrease (5 log10). In another study,[23] biofilms formed at 20 °C were more resistant to PAA than biofilms formed at 5 °C. The most effective sanitizer on contaminated stainless steel coupons was quaternary ammonium compound followed by PAA and chlorine. Low concentrations of ethylenediamine tetraacetic acid affect biofilm formation by inhibiting its initial adherence. Complete pathogen inactivation was obtained with a treatment of chlorine dioxide gas for 30 minutes against biofilms on slicers and peelers in a third study.[24] However, in a fourth study,[25] no sanitizer caused more than a 1.5-log CFU/cm2 reduction of Listeria when treated and untreated stainless steel or aluminum coupons that had been cut from a used deli meat slicer were compared. Additionally, no cleaning cloth-containing sanitizer produced more than a 1-log reduction compared with controls. It may be good to rotate sanitizers for various applications, including boot-dip stations for reentry into RTE areas.

Novel Interventions
A variety of natural GRAS (generally recognized as safe)-approved chemicals have been tested on different RTE products for their anti-listerial properties without any product quality deterioration. These include essential oils, cinnamon powder, apple skin extract and organic acids like ferulic and malic acids. In addition, EO water, intense pulsed light (IPL), combinations of ultraviolet (UV) and hydrogen peroxide, nisin and heat, and flash pasteurization with lauryl arginate ester have been shown to be effective hurdles. Ultrahigh pressure (UHP), IPL and pulsed electric field (PEF) are emerging processing technologies developed to enhance the safety while maintaining the fresh-like quality of food. However, UV light is not as effective as other treatments in destroying L. monocytogenes. Decontamination methods using gamma radiation, PEF and UHP target different loci in the cell, and thus synergy between these treatments against L. monocytogenes is worth pursuing. More targeted interventions as alternatives to the use of antibiotics or chemical decontamination in food supply systems are at the research stage. For instance, it has been shown that a high concentration of strain-specific bacteriophages in processing plants can successfully control pathogens. Pectin-based antimicrobial edible coatings on chilled or frozen roast turkeys can minimize risks of foodborne listeriosis. A coating of lytic enzymes attached to silica nanoparticles can selectively kill Listeria on lettuce.

Prevention and Control Measures
Prevention and control measures should be considered for Listeria spp., not just L. monocytogenes, and at every aspect of the farm-to-fork continuum.[26] These include:

•    Review practices during primary production to minimize the introduction of Listeria spp. into the processing environment

•    Design and maintain targeted programs, including cleaning and disinfection, for processing equipment and facilities to reduce the opportunity for the introduction, survival and multiplication of Listeria

•    Introduce hurdle technology strategies that reduce the numbers of Listeria present in RTE food and the potential for any surviving L. monocytogenes to multiply during storage of the product (shelf life)

•    Use microbiological testing to validate the effectiveness of listericidal processes, cleaning and sanitation programs, and to identify sources of Listeria spp. in the processing environment and the presence and level in the RTE food or ingredients

•    Take action when Listeria spp. are detected to remove or eliminate the source and minimize the risks to consumers of any products that may have been contaminated by these organisms

•    Educate and train all stakeholders so they understand the difference between safe and risky practices and how they can contribute best to prevention and control strategies; the range of actions includes testing raw milk used in raw-milk cheeses; periodic environmental sampling and microbiological testing; reviewing the cleanability of industrial slicing machines; providing the most appropriate sanitizers for processing equipment; supplying the safest food to at-risk consumers and auditing of food processors’ food safety systems

One of the big concerns is that the incidence of listeriosis in the U.S. has remained unchanged at 0.3 per 100,000 people since 2000 despite the improvements made by the RTE meat and poultry industry to do more testing, redesign equipment and reformulate products. One initiative will be to give more attention to the retail sector, since risk assessments show that the risk of contracting listeriosis is greater from retailed than from processor-packaged products. Both retailers and consumers have a responsibility to treat RTE food products as they are intended for use, but to allow some safety margins for consumer temperature abuse and a longer storage time.

The new Food Safety Modernization Act rule requires companies to include preventive controls for specific hazards.[27] These types of controls can be process controls (like Critical Control Points) or sanitation controls (e.g., sanitation of FCS). However, these controls need to be validated first using existing or new scientific studies. Once the controls are implemented, they will need to be monitored, with documentation of who will be responsible, how the process will be monitored and recorded, what happens if there is a problem and how monitoring will be verified. The same is conducted for corrective action—determining not only the steps that need to be taken when a parameter goes out of control, but also who is to be notified, who is responsible for correction, where and how it is recorded, etc. A recall plan must also be put in place, specifying responsible person(s) and steps of the plan from triggering event to food collection and disposal, including monitoring and verification. One of the ongoing concerns is the competence of third-party auditors to detect and report problems and of manufacturers to take action on these. In the long term, a multiagency, multidisciplinary approach is required to understand the burden of disease associated with this pathogen and how to better coordinate the resources to respond to contamination and illness events.[26] L. monocytogenes is a very challenging opponent to beat when zero tolerance is the goal, and there must be continual vigilance by the industry in recruiting the best prevention and control strategies for both processing and retail food operations. Continual research for novel technologies applied throughout the food chain from raw ingredients to long-term storage of the finished product will eventually contribute to a substantial decrease in the rate of listeriosis.  

Ewen C. D. Todd, Ph.D., is president of Ewen Todd Consulting and adjunct professor in the departments of food science and human nutrition and large animal clinical sciences at Michigan State University (MSU). He was the former director of the Food Safety Policy Center and the National Food Safety and Toxicology Center at MSU. At both of these centers, he coordinated research in microbiology, toxicology, epidemiology, risk assessment, social science and policy in the area of food safety, distance education programs and outreach in the community. He is active in the International Association for Food Protection and other organizations, and speaks and organizes symposia at national and international meetings. He is an editor-in-chief of the Elsevier Encyclopedia on Food Safety and associate editor for the Journal of Food Science and is a frequent reviewer of manuscripts submitted to several scientific journals. He has been, and is currently, an expert witness both for the plaintiff and defendant in foodborne illness cases. He is a graduate of Glasgow University with a B.Sc. in bacteriology and a Ph.D. in bacterial systematics.

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