Say the words Listeria monocytogenes in a room filled with ready-to-eat meat processors and you’ll quickly capture their attention. This deadly pathogen has resulted in too many illnesses, deaths, meat and poultry plus non-meat recalls and in some cases, business closings.

Listeria monocytogenes (Lm) is the form of Listeria most commonly responsible for infections. It is also one of the most virulent foodborne pathogens. Initially described in 1926, Lm wasn’t identified as a cause of foodborne illness until 1981.

During the late 1980s, Lm emerged as a problem in deli meats and other processed products, according to the US Dept. of Agriculture’s Food Safety and Inspection Service. (See chart of largest Lm recalls)

But USDA and the Food and Drug Administration worked with plants to improve their procedures and established a “zero” tolerance for the pathogen in ready-to-eat products. As a result, between 1989 and 1993 the rate of illness from Lm declined 44 percent.

In August 2011, an American Meat Institute bulletin stated that in 2010, Listeria prevalence in all ready-to-eat meat and poultry products was .28, down from .37 in 2009. This represents a more than 80 percent decline since 2000 (1.45 to .28), according to the Food Safety and Inspection Service, which includes information from all three FSIS sampling projects.

FSIS analyzed 3,153 and 8,704 samples for Lm in its ALLRTE and RTE001 sampling projects, respectively. Ten positive samples were observed in ALLRTE (0.32 percent positive), and 24 positive samples were observed in the in RTE001 samples (0.28 percent positive). FSIS analyzed 1,854 products for Lm in the RLm sampling program in 2010. The sampling results produced four positives samples, resulting in 0.22 percent positive rate.

Industry digs in

James Hodges, executive vice president of the AMI and president of the AMI Foundation, says of this accomplishment, “It was a combination of factors, but the overriding reason is the industry has worked very hard to reduce the prevalence rate of Listeria in RTE products. That has been accomplished by an intense attention to sanitation in the plants plus aggressive monitoring and testing of the environment in plants – those two things coupled with product reformulations and other microbial interventions that were largely designed to eliminate or retard the growth of Listeria in refrigerated storage.”

When FSIS first started testing for Listeria monocytogenes in 1998 industry was running an approximate 2.5 percent prevalence rate in products. Now, it’s at about 0.3 percent prevalence rate. “That’s a tremendous drop of more than 80 percent,” Hodges says.

There has not been a recall associated with a known outbreak of Listeriosis associated with RTE meat products since 2003, Hodges adds. The recalls that have occurred since then were the result of product being tested and not being held in the plant until test results were available, he continues.

Looking at the incidence rates of foodborne illness associated with Listeria from all products, it is very near the 2010 National Health Objective of .25, Hodges says. This is about one illness for every 400,000 population. “It has turned into a relative low risk, and in my opinion – the US meat and poultry industry has led that charge,” he adds.

Smithfield’s approach

Henry Morris, Smithfield Foods’ senior vice president of operations and engineering, says all of Smithfield Foods’ independent operating companies utilize a multi-pronged, systematic approach to controlling product post-lethality contamination with Listeria monocytogenes. This approach includes:

• Focused training programs for in-plant personnel regarding proper food handling and control of Listeria in Smithfield facilities.

• Plant sanitation systems.

• Adding antimicrobials in/on products and/or the application of a process which either prevents the growth of pathogens (i.e. freezing) or commercially sterilizes then prevents the re-introduction of Listeria (i.e. cook-in bag, high-pressure treatment).

• Verification of sanitation using both general microbiological counts and targeted Listeria environmental and product sampling programs.

Hurdle technologies in Smithfield’s plants specifically addressing Listeria include adding various antimicrobials (e.g. lactic acid, citric acid, lactate/diacetate, various natural extracts, liquid smoke, etc.); applying surface antimicrobials (e.g. lauric arginate, citric acid); the cook-in-bag process; fermentation processes; freezing; high-pressure treatments; low-water activity products (below 0.91); and post-lethality surface pasteurization.

Smithfield’s programs/initiatives and hurdle technologies have been very successful in battling Listeria in-plant as witnessed in the chart that follows, Morris says:

Percent Listeria prevalence in all ready-to-eat (RTE) meat and poultry products

Listeria is an environmental contaminant, as a general rule, Hodges says. “When we cook products such as deli meats and hot dogs, it destroys the Listeria,” he adds. “Where Listeria has been associated with our products in the past is during the slicing or packaging process where it may be present in the environment – on equipment, in drains or other places. And Listeria grows in refrigerated temperatures. So, it is possible that Listeria forms harborages in these facilities and you have to be diligent by aggressively testing for Listeria species and to seek-and-destroy these potential harborages. You have to do environmental testing to find it because you can’t see it in all cases with a physical inspection. Sanitation, first and foremost, is key.”

If a company tests products, it should hold the product until it gets results. “Most companies do some minimal amount of product testing, but the key here is monitoring and testing the environment because you’ll never be successful of testing the product only,” Hodges says.

A ‘one-two punch’

The US meat industry has succeeded in controlling Listeria in RTE meats primarily for two major reasons, says Dr. James Marsden, Regent’s Distinguished Professor of Food Safety and Security, Kansas State Univ., associate director of the Biosecurity Research Institute located at KSU and North American Meat Processors Association senior science advisor.

“First is the widespread use of ingredients that inhibit the outgrowth of Listeria, including buffered sodium citrate, sodium lactate and sodium diacetate,” he adds. “Second are the advances that have been made in post-process pasteurization technologies, including high hydrostatic pressure, thermal pasteurization and cook-in-bag processes. These eliminate any post-process contamination with Listeria.”

Another factor for this pathogen’s decrease is continuing advances in sanitation and environmental control measures. “RTE plants have never been cleaner,” Marsden insists. “The incidence of Listeria in RTE plants has been greatly reduced.”

Academia has also played a role in this reduction by conducting validation studies for emerging control technologies, including high hydrostatic pressure (HHP) and secondary inhibitors, he adds.

“FSIS set the stage for these improvements through the regulation on Listeria and the establishment of the three alternatives for Listeria control,” Marsden continues. “Incentives were offered to plants that chose to operate under Alternative I and Alternative II. These incentives helped drive companies to implement improved Listeria control measures. Of course, the HAACP regulation also played a role.”

This is a success story not widely recognized and is almost unparalleled in modern history, Hodges says.

“A lot of credit goes to individual companies, industry associations, academia and the research community, among others. We want as many tools in the toolbox as you can get for controlling Listeria. Then it’s up to each individual company to apply those interventions they deem work best for their operation,” he concludes.