In April 2007, the U.S. Centers for Disease Control (CDC) released preliminary data from its FoodNet surveillance system for 2006 that show significant declines in the rate of foodborne infections associated with Campylobacter, Listeria, Shigella and Yersinia and increases in illnesses attributed to Escherichia coli and Vibrio infections (Figure 1). Certainly, the less-than-stellar news on the E. coli front isn’t surprising, given the nationwide outbreaks linked to fresh produce, specifically spinach, tomatoes and lettuce, in 2006; but perhaps disappointing given the heartening declines that occurred between 2003 and 2004 when the beef industry made great strides in implementing then-new U.S. Department of Agriculture (USDA) food safety measures in processing facilities. Also of concern is the fact that, according to CDC, the number of Vibrio infections rose to its highest level since FoodNet surveillance began in 1996.

Why, then, is this article focused on Listeria monocytogenes (LM) control when the CDC’s latest foodborne illness incidence data indicates that the estimated decline in the rate of foodborne infections attributed to this pathogen is 34%, a percentage reduction that seems fairly substantial? For a few important reasons:

1. Because, as the agency notes in its findings, when compared with the 1996-1998 baseline period figures, significant declines occurred in the estimated incidence of Listeria—but most of these declines occurred before 2006. In its summary, the agency states, “In 2006, the incidence of Listeria infections remained higher than at its lowest point in 2002.” Thus, although the estimated incidence rate of listeriosis fell by 34% and accounts for just 138 cases reported in 2006 (as compared with Salmonella, which accounted for 6,655 cases [39%], 5,712 Campylobacter cases [33%] and 590 cases of Shiga toxin–producing E. coli O157, for example), the fatality rate associated with L. monocytogenes infection is nearly 25%, well above any other foodborne pathogen.

2. Because, even though of the 17,252 laboratory-confirmed foodborne infections in 2006 identified in the FoodNet surveillance, Listeria infections account for just 138 cases of foodborne illness, CDC notes that this number only represents reported cases. The agency estimates that for the total number of reported or verified case of listeriosis, the truer, extrapolated number of food poisoning cases caused by L. monocytogenes may be as high as 2,500 illnesses and 500 deaths annually.

3. Because, the 2006 FoodNet statistics show the number of Listeria cases is currently 0.31 per 100,000 people, which while approaching the nation’s Healthy People 2010 objective to reduce the number of listeriosis cases to 0.25 cases per 100,000 people, is evidence that there is more work to do to achieve this goal (Table 1).

Although the increased incidence rates reported for other foodborne pathogens in the 2006 FoodNet data should not be discounted, the fact is that L. monocytogenes contamination remains a significant challenge to the food industry for the reasons above and more. The challenges industry faces in controlling this damp-loving bug in processing plants and food handling establishments are numerous, including:

• L. monocytogenes is ubiquitous in the natural environment, found in soil, water, and vegetation, and is carried by healthy animals. We can swab or sample virtually any damp place in the environment and find it.

• L. monocytogenes is easily transferred from floors and walls or other environmental surfaces onto equipment where it finds harborage points and can then be transferred directly to foods.

• L. monocytogenes can survive and grow in refrigerated minimally pasteurized and packaged ready-to-eat (RTE) products, at temperatures as low as 28F, as well as in low-oxygen conditions.

• Similarly, although L. monocytogenes can often be present in raw foods, it also can be present in cooked foods due to post-processing contamination.

• L. monocytogenes can survive for long periods of time in the environment, on foods and on processing plant surfaces, as long as some form of moisture is available.

In fact, most of the growth and survival control techniques used to protect foods from pathogen contamination in the processing environment do not necessarily work for Listeria as well as we typically expect. Refrigeration, vacuum packaging, acidification—none of it guarantees that we’ve eradicated the problem or prevented Listeria from reappearing even years after we think we’ve overcome it. Ultimately, its ubiquity in the environment, its ability to grow in a normal range of characteristics that we normally expect to keep foods safe, and its resistance to succumb to a variety of control measures in certain at-risk foods, make L. monocytogenes a very difficult pathogen to control in the food processing environment.

This is why U.S. regulatory agencies, industry and researchers continue to work to improve methods that reduce the risk of Listeria contamination in the food supply chain. Both the U.S. Department of Agriculture’s Food Safety and Inspection Service (USDA FSIS) and the U.S. Food and Drug Administration (FDA) have developed important information and guidelines in this area. As the only federally mandated rule that requires processing facilities to comply with Listeria control measures within a specific framework, 9 CFR 430.4, Control of Listeria monocytogenes in post-lethality exposed ready-to-eat products, has been implemented in specified ready-to-eat meat and poultry product processing plants since October 2003. As we approach the fourth year of implementation, many insights have been gained by and resources developed for RTE meat and poultry product manufacturers to aid in the battle to reduce Listeria risk factors in the plant.

In addition, the knowledge gained offers a framework for RTE product manufacturers outside the oversight of FSIS to enhance their Listeria control measures and systems—particularly those operations that make higher risk products (Table 1). L. monocytogenes contamination has effected product recalls across RTE food categories. In the past two years, FDA Class I recalls for products testing positive for or linked to Listeria included potato and egg salads, frozen strawberries used as a primary ingredient for smoothies, raw milk, cole slaw, packaged fresh-cut fruit, prepackaged turkey sandwiches, smoked salmon and trout, radish, alfalfa and bean sprouts, and various chicken, turkey, vegetable and imitation seafood-flavoredspreads and dips. USDA FSIS L. monocytogenes recalls since January 2007 have involved hot dogs, sausage, deli-style ready-to-eat pork, roast beef, chicken and turkey products, hog head cheese, spicy Thai-style pasta salad with chicken breast, ham salad and dried beef, and ready-to-cook chicken breast strips.

The FSIS L. monocytogenes regulation and guidance materials are useful for any company manufacturing RTE foods that can support the growth of Listeria. These include foods with a water activity greater than 0.92, or a pH greater than 4.39. As indicated earlier, Listeria is a hardy switch-hitter that grows aerobically and anaerobically at low temperatures thereby frustrating the more commonly accepted methods of keeping food safe. When it comes to Listeria, anyone who is in the business of manufacturing high- or medium-moisture foods has a shot at becoming famous the wrong way. Here, we’ll discuss some of these resources and industry learnings within the framework of the FSIS rule to see where we are now in the progression toward reducing Listeria contamination of foods and hopefully, move us closer to achieving more significant declines in future FoodNet surveillance reports.

Background: Getting to Where We Are
In June 2003, FSIS made final its interim rule for the control of L. monocytogenes on certain RTE meat and poultry products (9 CFR 430.). As of Oct. 6, 2003, the rule required that certain RTE meat and poultry processors implement one of three risk-based alternatives for categorizing their products, develop written programs to control L. monocytogenes, and verify the effectiveness of those programs through testing. The rule also requires RTE product processors to share plant information and testing data with the agency, and emphasizes the importance of incorporating a multiple hurdle strategy comprised of a combination of microbial interventions in the RTE processing environment.

Once a manufacturer has established that the plant is producing RTE meat or poultry products that fall under the FSIS rule, the establishment must select one of the following three intervention alternatives to control for L. monocytogenes:

• Alternative 1. Employ both a post-lethality treatment and a growth inhibitor (antimicrobial agent or process) for L. monocytogenes on RTE products (Figure 2). Establishments opting for this alternative are subject to FSIS verification activity that focuses on the post-lethality treatment’s effectiveness. Sanitation is important but is built into the degree of lethality necessary for safety.

• Alternative 2. Employ either a post-lethality treatment or a growth inhibitor for the pathogen on RTE products (Figure 3). Establishments opting for this alternative are subject to more frequent FSIS verification activity than those in Alternative 1.

• Alternative 3. Employ sanitation measures only (Figure 4). Establishments opting for this alternative will be targeted with the most frequent level of FSIS verification activity. Within this alternative, FSIS places increased scrutiny on operations that produce hotdogs and deli meats because they are high-risk products for listeriosis as identified in the 2001FSIS/FDA risk ranking.

Since the implementation of the rule began, many larger RTE food processing operations have invested in one or both of the first two alternative programs, which require that processors validate the intervention treatments and provide supporting scientific data as evidence of the efficacy of those interventions to FSIS. The FSIS LM Compliance Guidelines include a draft form where companies are required to report their production volumes, as well as to identify their product category, process alternative, the log reduction or minimum log increase of the relevant interventions, and the frequency of establishment testing. The agency uses this information to develop its verification sampling program for each RTE processing operation.

One of the keys to meeting L. monocytogenes risk reduction aims and to identifying the appropriate scientific resources required is to truly understand the treatment requirements involved. Essentially, Alternative 1 requires that both treatment types are met, and Alternative 2 requires that either of the two treatments is met. The following definitions of each type of treatment further elaborates differences in these requirements:

Alternative 1: Post-lethality Treatment (PLT):
• Must cause a reduction in LM over time.

• Must be a CCP in the HACCP plan.

• Safe Harbor = 0.85 aW or pH 4.39 requiring verification only in the HACCP plan. Higher numbers must have specific validation.

• All other PLT methods must have specific validation.

• Safe Harbor = Plant has documentation showing that the intended effect occurs prior to distribution of the product into commerce.

• No requirement for product contact surface (PCS) testing. It is recommended but not required.

Alternative 2: Antimicrobial Agent or Process (AMAP):
• Must restrict LM growth over time.

• Could be a CCP, SSOP, or prerequisite program.

• Safe Harbor = 0.92 aW or pH 4.39. Higher numbers must have validation. Documentation must show that the water activity is met, but does not need to be a CCP.

• All other AMAP methods must have specific validation.

• PCS testing required at least quarterlywith provisions for product hold and test if positives are found.

Alternative 3: Control by Sanitation Only:
• Rigid requirements for sanitation procedures and handling practices.

• Prescriptive PCS sampling based on product type and plant size.

• Procedures for product hold and test if positives are found.

As defined by FSIS, a post-lethality treatment is a kill step that is applied or is effective after post-lethality exposure. It is applied to the final product or sealed package of product in order to reduce or eliminate the level of pathogens resulting from contamination from post-lethality exposure. An antimicrobial agent is defined as a substance in or added to an RTE product that has the effect of reducing or eliminating a microorganism, including a pathogen such as L. monocytogenes, or that has the effect of suppressing or limiting growth of L. monocytogenes in the product throughout the shelf life of the product. Examples of antimicrobial agents added to RTE products are potassium lactate and sodium diacetate. The manufacturer also may opt to use an antimicrobial process, which is defined as an operation, such as freezing or procedures for pH or water activity control, applied to an RTE product that has the effect of suppressing or limiting the growth of microorganism such as L. monocytogenes, in the product throughout the shelf life of the product.

Again, the key difference is that for the Alternative 1 post-lethality treatment it must be bacteriocidal, you must show that you have killed pathogen and prove that you’ve killed it before product is shipped. For an Alternative 2 antimicrobial agent or process, you must provide evidence that the process is bacteriostatic; i.e., that you’ve hindered or stopped the growth of L. monocytogenes.

Thus, the major thrust of 9 CFR 430 and related compliance and guidance materials focuses on moving the RTE industry toward validating these interventions to reduce the levels of L. monocytogenes. Given this, the RTE food manufacturer is charged with finding adequate intervention validation data for the process controls selected. The question is, where do we find these resources? Nearly four years after implementation of the rule, regulated sectors of industry have found a few key methods for gathering and documenting the needed data, which may assist other food manufacturers instituting L. monocytogenes risk reduction programs.

Getting Resourceful
The FSIS “Updated Compliance Guidelines to Control LM in Post-Lethality Exposed Ready-To-Eat Meat and Poultry Products” is the first resource that an RTE food manufacturer, whether or not they are in the meat or poultry sectors, should consult. Key data in the guidance comes from a compendium of tables published by the International Commission on Microbiological Specifications for Foods (ICMSF), Handbook 5. The ICMSF book (Characteristics of Microbial Pathogens) is also of good value for establishing temperature related growth and death values for different organisms, as well as a wealth of other data useful for validating your process. The FSIS Compliance document goes into great detail on the Alternatives and the requirements to use, validate and document those process controls. Another helpful aspect of the Compliance Guideline is the additional information found in the appendices about specific post-lethality treatments and antimicrobial agents and processes.

One of the most useful documents available to RTE product manufacturers is the FSIS Procedures for the Evaluation of Establishment Control Programs for LM (LM Checklist Guidelines), a question-and-answer formatted document that a FSIS Enforcement, Investigations, and Analysis Officer (EIAO) or your company’s own evaluator should use when doing a comprehensive food safety assessment. The form is available as a PDF for download from the FSIS website, and is also included in the FSIS Compliance Guidelines described above. It is particularly helpful when printed out and the food safety team discusses and completes the checklist-style questionnaire as they would when developing or updating a HACCP manual. Ultimately, if you can’t answer the questions and thus arrive at a numerical score that helps indicate what process controls should be instituted, you’ve got some serious evaluation of your program to do. Without this score, you are unlikely to do well in an actual FSIS Comprehensive Food Safety Assessment, and what you believe are the strengths of your Listeria control program may in fact be weaknesses and therefore a liability to your company.

The LM Checklist Guideline begins with a series of preliminary Yes/No questions to help you select the appropriate evaluation section to complete (i.e., choose Alternative 1, 2 or 3 ). For example, if you check “Yes” for the question “Does the establishment produce post-lethality exposed ready-to-eat product covered by 9 CFR 430?” you move on to the next question. If you check “No,” the instruction says you can stop the assessment for this product since the product is not covered by 9 CFR 430. If you check “Yes” to the second question, “Did the establishment develop control measures that meet one of the three Alternatives for the product, as required in 9 CFR 430.4?” you move onto the next step, which is to list the chosen Alternative for that product(s).

Once you’ve answered the preliminary questions, you can move on to the simple Yes/No forms provided for each Alternative and complete the specific checklist for your product. Based on the total number of “Yes,” “No,” “Not Sure,” and “N/A” responses tallied, a score is determined as to whether the validation and documentation of the process control for that product can be characterized as “conclusive,”, “substantiated,” or “inconclusive.” Questions at this stage include those that prompt general responses (Is the post-lethality treatment validated and documented?), as well as more detailed answers (Do the information in the HACCP plan, Sanitation SOP and Prerequisite programs [e.g., Alternative, post-lethality treatment, AMAP, log reduction, log suppression, FCS testing frequency, etc.] corroborate the information on the survey form that the establishment submitted?). Questions are followed by notations about what kind of supporting evidence or documentation the company possesses to validate its claim that this Alternative will in fact provide the required L. monocytogenes log reduction (Alternative 1) or log suppression (Alternative 2.).

Once you’re given the score to this set of questions (again, based on the total number of “Yes,” “No,” “Not Sure,” and “N/A” responses tallied, a score rates whether the validation is “conclusive,”, “substantiated,” or “inconclusive”), you move into the final set of questions, where points are assigned from which results a total assessed value. This section is in an easy-to-use tabular form, which provides scores based on the method of validation and the log reduction or log suppression achieved by the Alternative used. As the form states, the more rigorous the validation method and the log goals achieved by the selected treatment, the lower the risk and the higher the scores (i.e., the risk of L. monocytogenes contamination goes down as the score goes from inconclusive to conclusive). Now, you can take the results from the previous section of questions and circle the corresponding numerical score provided in the table (Table 2). FSIS provides an example describing how to do this, such as the following, which appears for the Alternative 1 post-lethality treatment section: “If the establishment’s PLT as documented in its HACCP plan was derived from a manufacturer challenge study and achieves 2 log reduction of LM, and the result from Section I is “conclusive,” circle the score provided on the appropriate row (manufacturer challenge study and equal to or greater than 2 log reduction), which in this case is 10.”

Finally, the assessment guide provides another simple Yes/No-style form to help the processor or FSIS auditor to evaluate and provide documentation for the ongoing verification system that will be used for the selected alternative. Again, there is a rating and scoring system that results in a more objective idea of the quality of the verification activities to be performed.

A Resourceful Example
Let’s look at a typical way in which an RTE food manufacturer conducts a L. monocytogenes hazard reassessment for its dry and semi-dry fermented meat product operation in order to discuss what resources we can better utilize to meet documentation or validation requirements for Alternative 1 or 2 process controls—and where we might find them. In this example, the processor finds that, based on the available science, L. monocytogenes will die over time if introduced to the products. The company must now provide documentation to FSIS showing the veracity of this statement.

Statement: It has been demonstrated scientifically that certain foods will not support the growth of L. monocytogenes including foods that are maintained at temperatures of -1C or below, foods that have pH values of less than 4.4, and foods that have water activity values of less than 0.92. All products produced and/or used at this establishment have documented water activities less than 0.92 and therefore will not support the growth of L. monocytogenes. According to all available scientific data this water activity level will cause L. monocytogenes to rapidly die if introduced to the product in the post-process environment.

Supporting Resources: Here, the company can look to FSIS documents for some of the supporting evidence. For example, on pg. 13 of the FSIS L. monocytogenes Compliance Guidelines, the processor can cite the International Commission for the Microbiological Specification of Foods (ICMSF) Characteristics of Microbial Pathogens data and find that dry and semi-dry fermented meat products do not meet the water activity, pH and temperature maintenance criteria to the level of an at-risk food as per the Listeria rule.

Statement: To meet the requirements of Alternative 1, FSIS guidance has defined the dehydration of meat products as being an antimicrobial process that also is a post-lethality treatment if the water activity is 0.85 or less, this measurement is a CCP in the HACCP plan, and the product is held under establishment control until the post-lethality effect can to take place. The company elects all products to be classified as Alternative 2 for the approximate week it takes for the 1 log reduction to occur.

Supporting Resources: As stated, the manufacturer will first look to the FSIS definitions of the Alternatives in the Compliance Guidelines for supporting data. The agency lists the safe harbors for this pathogen such as water activity and pH limits, which are proven to stop the growth of Lm. The manufacturer need only state that these criteria are met for supporting documentation of an antimicrobial agent or process. For post-lethality treatments, the manufacturer must show this as well, but must also explain why this particular process control is being used and provide the source(s) of the supporting scientific validation data.

Of course, in our example here, this is easy to determine because of the inherently low water activity required for shelf stable meat snacks. However, what if the product is deemed high risk and the processor wants to determine how to make a safer product through the application of either a PLT or an AMAP? For those manufacturers, there are three essential sources of scientific supporting data to find applicable process control and treatment information:

1. Peer-review Literature. FSIS says that if a company has a scientific reference to its particular process and product, they can use that reference as their validation. Indeed, published peer-review articles provide high credibility to the RTE processor’s claims of validated process controls for L. monocytogenes. Univesities, research institutions, food industry trade organizations and government agencies have been conducting more and more challenge studies on the validation of food safety interventions. Many of these studies focus on the types of post-lethality and antimicrobial agents/processes that will or potentially can reduce the incidence of L. monocytogenes throughout the food supply and distribution chain.

When searching out supporting data to validate interventions, RTE manufacturers should review as much of the available scientific literature as possible. Industry-specific trade associations, such as the National Meat Association and the American Meat Institute Foundation, as well as scientific organizations like the National Center for Food Safety and Technology at the Illinois Institute of Technology, and the International HACCP Alliance, offer much in the way of published and peer-review articles, research studies and pilot plant activities regarding detection, prevention and control measures for L. monocytogenes in a variety of RTE products. Among others, the International Association for Food Protection’s peer-review journal, the Journal of Food Protection, is another good example of where to find credible published articles that can be used to validate your chosen Listeria treatment or process.

These organizations and industry consulting firms also conduct and sponsor seminars and workshops where processors can obtain references and other resources as to the efficacy and application of various intervention strategies and technologies from experts in the field.

2. Intervention Technology Suppliers. Also look to the post-lethality technology and antimicrobial treatment/agent manufacturers for assistance when gathering supporting validation data. These suppliers have in-depth knowledge about how their systems work and the effectiveness of their technologies in particular products, production processes or processing environments. In many cases, these companies have invested in third-party or university research projects that support the veracity of their claims.

3. Challenge Studies. Especially as evidence pertains to validation, FSIS personnel are always interested in where the data came from in order to judge its level of credibility. Microbial modeling program data might include some variability or the computational system cannot allow for factors that affect the conclusiveness of the results. Challenge studies, by comparison, are considered the best source of and the most credible validation evidence, since the results are generated from sampling your specific product and processes. This data about the effectiveness of Listeria interventions can be generated in house or by working with a food industry or university laboratory that has experience in RTE product testing and that can provide the caliber of microbial challenge studies that will validate the process and the process control or treatment.

Statement: Dry and semi-dry fermented products and jerky have never been definitively linked nationally or internationally to listeriosis.

Supporting Resources: The first resource to use is the 2003 FDA/USDA Quantitative Assessment of Relative Risk to Public Health from Foodborne Listeria monocytogenes Among Selected Categories of Ready-to-Eat Foods, which as mentioned earlier, categorizes several food categories by level of potential for L. monocytogenes growth and survival. In Tables II-4 and II-5 of this resource outbreaks of listeriosis with known food vehicles are listed nationally and internationally for the years 1970-2002. There are no outbreaks associated with dry/semidry fermented sausage or jerky in these references.

Ultimately, our example RTE manufacturer can conclude that since its products can be verified to be in a water activity range that will not support the growth of L. monocytogenes and will in fact cause the pathogen to die, and there is no epidemiological data nationally or internationally to support the concern that dry or semidry fermented products or jerky have been sources for listeriosis, that a public health incident from L. monocytogenes in its RTE dry and semi-dry fermented meat products is not a hazard reasonably likely to occur. And the processor can do so with a high level of confidence that the process controls put in place satisfy compliance requirements and are actually keeping foods safe.

Getting Ahead of Listeria
The food industry understands that Listeria is constantly moving around and being reintroduced into the environment. If we’v