While many different pathogens can cause foodborne illnesses, there are only a few pathogens for which sources within food processing and handling environments have been directly linked to foodborne illness cases and outbreaks. Listeria monocytogenes, one of the organisms most troubling to refrigerated and frozen food producers, is a particularly problematic pathogen in this regard.
Listeria monocytogenes is mainly found in moist conditions, including soil, surface water and decaying vegetation. It is also salt-tolerant and can withstand and grow in temperatures below 1°C, making it an extremely survivable pathogenic organism for nearly any processing environment, including those that process raw materials using colder temperatures. Contamination can come from these raw materials, from the air and from food workers and food processing environments.
Processors generally rely on processing “kill steps” like heat pasteurization, high-pressure processing (HPP) or radiation processing to prevent the presence of pathogens like Listeria in finished products. However, most experts agree that post-processing contamination from food contact surfaces represent the most obvious danger, particularly because surfaces can harbor the organism for years.
While “L. mono” is not one of the most common causes of foodborne illness, it is among the leading causes of death from foodborne illness. The Centers for Disease Control and Prevention (CDC), Washington, D.C., estimates that, in the United States, domestically acquired foodborne L. monocytogenes results in 255 fatalities per year. These numbers have declined by nearly 50% since the year 2000, when the CDC estimated approximately 500 deaths resulting from the pathogen in the United States per year. However, while deaths are down, increased regulatory surveillance on the pathogen means that the frequency of both Listeria detection and recalls due to Listeria are on the rise.
Threats, monitoring and detection
L. monocytogenes is most commonly associated with raw or ready-to-eat foods (RTE), including RTE processed products, poultry and meats, vegetables, seafood, cheeses (particularly soft varieties), unpasteurized milk and ice cream.
Several high-profile L. monocytogenes outbreaks over the past decade have plagued food companies and their customers, drawing more concern and attention from inspectors and sparking a “Listeria hysteria” among the media and the general public.
Control of Listeria species, including Listeria monocytogenes, is vital. Food processors must create well-thought-out environmental sampling plans and methodically sample finished product and key zones within food production environments as well as other contact surfaces in plants that could potentially be reservoirs for Listeria. They also must be mindful of the fact that L. monocytogenes can grow in refrigerated temperatures, so areas often regarded as non-problem areas for other pathogens due to their temperature need additional attention.
Environmental sampling is increasingly considered an essential part of any comprehensive food safety program, but building a program and achieving consensus internally is easier said than done. Producers have historically lacked the resources to design and implement comprehensive and effective environmental monitoring programs to help address the threat of post-processing contamination.
That’s why rapid test kits for Listeria species as well as the Listeria monocytogenes pathogen are among the fastest growing tests in the food industry. As global regulatory requirements become more stringent, the demand for Listeria testing dramatically increases. And, as the food industry becomes increasingly competitive and regulations become more sophisticated, processors need results faster and with a higher degree of precision.
Earlier this year, the U.S. Department of Agriculture’s Food Safety and Inspection Service (USDA FSIS) revised its Microbiology Laboratory Guidebook (MLG), the official guide to the preferred methods the agency uses when testing samples collected from audits and inspections. Notably, the MLG updated the rapid methods for both Salmonella and Listeria monocytogenes to embrace a new pathogen detection innovation, namely loop-mediated isothermal amplification, or LAMP technology, which provides an alternative to traditional PCR tests and offers equivalent or better results in a shorter time, with fewer steps.
However, even if a processor has the best technologies and monitoring programs in the industry, the human aspect of Listeria detection and prevention cannot be understated. This is true not only when it comes to training specific sampling, processing and pathogen testing techniques and practices, but also in motivating employees to maintain strong personal hygiene practices and to cultivate a culture of food safety.