Control of Listeria monocytogenes in ready-to-eat foods through environmental monitoring: perspectives from the 2017 FDA draft guidance document on “Control of Listeria monocytogenes in ready-to-eat foods”

The genus Listeria is ubiquitous in the natural environment (i.e., soil, vegetation and surface water) and most Listeria species are not pathogenic to humans. Listeria monocytogenes is the only species in the Listeria genus of public health significance. Not only is L. monocytogenes able to infect more than 40 species of animals and birds but is also commonly shed in the feces of asymptomatic production animals, including cattle and sheep. L. monocytogenes can cause a severe invasive disease known as listeriosis in high-risk host populations, including pregnant women and their neonates, the elderly and otherwise immunocompromised individuals (e.g., cancer and HIV patients). L. monocytogenes infections account for approximately 20% of deaths attributable to foodborne pathogens in the U.S. per annum.

Because Listeria is common in the general environment and commonly shed by asymptomatic animals, the organism may be found in raw materials and other ingredients used to manufacture and process ready-to-eat (RTE) foods. Although Listeria is readily controlled by lethality treatments such as cooking, pasteurization, and drying, exposure to the environment after a lethality treatment (prior to packaging) can lead to cross-contamination of RTE food products from environments where foods are manufactured, processed, packaged and held. Combined microbiological testing and molecular subtyping studies have demonstrated utility in elucidating Listeria transmission patterns in RTE food processing plants and identifying harborage sites within a facility.

Additionally, these studies have demonstrated that a given facility tends to be predominated by one or a few specific Listeria DNA fingerprint types. Whole genome sequencing studies have shown that Listeria can persist in the environment of food processing facilities for years while undergoing minimal genetic change. In addition, unlike other foodborne pathogens, Listeria is capable of growing slowly during refrigerated storage throughout the shelf-life of ready-to-eat foods. Trend analyses of combined testing and molecular subtyping data along with in-plant trainings on Listeria ecology have been shown to improve knowledge and may lead to better Listeria control in the environment.

In 2017 the FDA released a new draft guidance document on “Control of Listeria monocytogenes in ready-to-eat foods”, which replaces the 2008 document. The purpose of the document is to help companies subject to the current good manufacturing practices and preventive controls rule in 21 CFR part 117 in accordance with the Food Safety Modernization Act. USDA:FSIS industry best practices and the “seek and destroy” philosophy have been incorporated into the 2017 FDA guidance document to harmonize the regulatory approach to control L. monocytogenes in RTE foods and benefit facilities regulated by both agencies. The document includes “non-binding” recommendations to control L. monocytogenes involving personnel, plant design and operation, equipment cleaning and maintenance, sanitation, raw materials and other ingredients, lethality treatments and product formulations to inhibit the growth of L. monocytogenes as well as storage and transportation practices. The overall goal is to eliminate L. monocytogenes contamination or control the growth of L. monocytogenes in finished RTE products through controls on raw materials and other ingredients, listericidal controls, listeristatic formulations, separation of raw and cooked food areas, environmental sanitation, sanitary equipment design and physical barriers to prevent cross-contamination of the finished RTE product by the environment.

Of note, the document outlines strategies for risk-based environmental monitoring plans, including the zone system to characterize areas within a facility based on risk for cross-contamination of RTE product exposed to the environment following the lethality step. The location and number of environmental sites to be tested should be clearly described in the environmental monitoring written procedures (e.g., an extensive list with random selections to be exhausted within a month or an appropriate number based on the plant size). The guidance document recommends testing environmental samples for Listeria spp. rather than L. monocytogenes because a Listeria spp. positive result would include L. monocytogenes and represent a site where L. monocytogenes could be harbored. One of the big changes comparing the 2017 document to the 2008 document is that a facility does not have to react to a Listeria spp. environmental positive as if it were a L. monocytogenes positive. The document indicates Listeria spp. is expected to be detected in the environment occasionally. The document does not recommend compositing environmental samples from food contact surfaces and if compositing is performed no more than five samples should be composited for testing purposes. If a site consistently tests negative for Listeria spp., it is recommended that site be removed from the sampling plan and replaced with new site(s) to identify other potential sources of contamination.

It is recommended to collect environmental samples several hours into production or ideally at the end of production just prior to cleaning and sanitation. It is also recommended that the FDA procedure for environmental samples (or equivalent method) be used for testing and that the laboratory performing testing is able to perform the most current and valid testing methods (ISO 17025). Compositing is not recommended for follow-up environmental investigations to locate specific harborage sites. Risk-based corrective actions should be taken when a sample is positive, depending on product type (i.e., whether or not the product supports the growth of L. monocytogenes) and environmental sample type (i.e., food contact surface vs. non-food contact surface). Corrective actions can range from intensified cleaning and sanitation, intensified sampling and testing, root cause analysis, to “hold and test” of the affected RTE product). The document recommends that if a FCS is positive for Listeria spp., further testing should be conducted to determine if the sample contains L. monocytogenes. Finally, the document recommends testing RTE food products, for L. monocytogenes, rather than Listeria spp. due to public health implications. Specific recommendations for sampling of various environmental sites (e.g., swab vs. sponge, testing small parts such as bolts, and how to handle liquid samples) as well as guidance for “hold and test” of RTE food products provided in the document.

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