What to Know about Listeria

Listeriosis, an infection caused by ingestion of Listeria monocytogenes, is especially dangerous for certain at-risk populations, including pregnant women, fetuses, infants, the elderly and those battling chronic diseases. According to the Centers for Disease Control and Prevention (CDC), while the rate of listeriosis has fallen by 40 percent in the past five years, Listeria monocytogenes still accounts for almost 500 deaths each year.

Listeria monocytogenes, unlike other foodborne pathogens, survives and grows at refrigeration temperatures in many food items including dairy products and deli meats.

Listeria is a ubiquitous microorganism and has been found in food manufacturing plants with a higher incidence in seafood, fresh vegetables, and raw milk containing foods. Minimizing Listeria using different control applications to the extent that current technology can achieve is a major goal for food safety plans. Since ready-to-eat foods are particularly susceptible, raw materials, which carry high loads of organisms, are routinely sampled and tested for the presence of Listeria in order to prevent cross-contamination inside a plant. Testing for generic Listeria as an indicator of the potential presence of Listeria monocytogenes improves its detection prior to distribution into commercially produced foods.

Classical microbiology methods utilizing Modified Oxford Agar (MOX) and PALCAM agar plate counts are listed in the FDA Bacteriological analytical manual (BAM) and the USDA/FSIS Microbiology guidebook for the detection and enumeration of Listeria. The increased need for procedures that provide simple, easy, reliable and affordable methods for Listeria detection are starting to replace elaborate traditional methods. Until recently, the major obstacle in offering improved testing methods was gaining approval from regulating/validating agencies, e.g. FDA/ BAM, USDA/FSIS and AOAC. However, any of these rapid tests are now being considered standard methods and/ or are in the process of receiving validation approval.

The clinical industry has widely used latex agglutination kits and prepared media, both of which eliminate manual preparation and reduce identification time. Latex agglutination products are quick, simple means for identifying a positive or negative presence of Listeria. The technology involves manufacturing a complex list of antibodies or antigens against the Listeria bacterium which are then coated onto latex beads. If the test sample has the corresponding antigen or antibody when mixed with latex beads visible clumping (agglutinatation) can occur, indicating presence of Listeria. If the sample does not have the corresponding antibodies or antigens no clumping will occur, indicating that Listeria is not present.

Chromogenic media are growing in popularity due to the beneficial time and cost savings of using a single plate method when performing tests on food products, raw materials and environmental samples. Chromogenic agar is a selective and differential media enfolded as one media for the isolation and identification of specific genus and species. BBL CHROMagar Listeria is listed in the FDA/BAM manual as a recommended method and has received AOAC-RI approval for the confirmatory identification of Listeria monocytogenes/ivanovii.

The addition of selective agents inhibits the growth of gram-negative organisms, yeast and fungi while the added chromogen substrates differentiate species by visual characteristics. For example, BBL CHROMagar Listeria grows all Listeria as blue-green colored colonies, while a specific enzyme found in L. monocytogenes/ivanovii acts upon the phosholipid substrate in the media producing an opaque white halo around the blue-green colonies. In addition to providing one-step plating, time to results from primary enrichment is as little as 24 hours.

Presently, rapid biochemical identification kits are used for additional species identification, requiring a minimum of 24 hours for reliable results. When comparing 24 hour rapid tests, it is important to consider additional steps required, which increases time to confirmation. The advances made in the past few years with rapid testing products provide easy efficient methodologies for sampling Listeria with greater levels of specificity and selectivity than traditional methods.

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