There are also a couple of myths associated with L. monocytogenes testing that are important to dispel:
Myth #1: Listeria testing cannot be done in house
The food industry has many available technologies for the detection, isolation, identification and characterization of L. monocytogenes. Rapid and automated microbiological methods have provided tremendous advances for the food industry. Because Listeria contamination of foods results primarily from resident strains in food processing plants, it is ideal if these specific biotypes can be identified. This is achievable for large companies with a trained technical staff to perform pulsed-field gel electrophoresis (PFGE) or ribotype analysis.
Distinguishing transient biotypes, which may be associated with raw foods, from resident types implicated in human illness, allows manufacturers to focus efforts to eliminate niches. While this may be a practical approach for large manufacturers, it is much more cost-effective for smaller plants to target Listeria as indicator organisms and use this data to achieve environmental control. EL plates are ideal tools for routine environmental testing in verification of cleaning and sanitation efforts.
Depending on the level of expertise in the food company and how the facility is designed for production, specific testing for L. monocytogenes is not normally advised within the plant environment. This is because positive controls are required in testing and the potential exists for laboratory contamination or introduction of L. monocytogenes into the processing environment. However, with EL plates, the nonpathogenic species L. innocua can be used as a positive control, thus eliminating the need to use the pathogenic L. monocytogenes. If follow-up is desired, EL plates can be shipped to an outside lab for further analysis and confirmation of biotypes resulting from growth on the EL plates.
Myth #2: Non-enrichment methods are not as effective as enrichment methods
The advantage of the EL plate is that it is a no-enrichment, sample-ready culture medium with a chromogenic indicator, providing results within 29+2 hours. A comparative study of the EL Plate Method was conducted versus USDA and modified USDA procedures for recovery of Listeria from 192 environmental and food contact surfaces (Groves and Donnelly, 2005). The USDA procedure employs primary selective enrichment in the University of Vermont Medium (UVM) followed by secondary enrichment in Fraser broth and selective plating on MOX agar.
Previous laboratory results found that the sensitivity of the USDA method could be improved through dual primary enrichment in both UVM and Listeria repair broth (LRB) media (Pritchard and Donnelly, 1999). LRB is a highly nutritious repair/ enrichment medium which supports both repair and high level growth of Listeria. In their studies on enrichment of dairy environmental samples in the UVM and Listeria repair broth, combining these two primary enrichment media into a single tube of Fraser broth for dual secondary enrichment yielded a significantly higher percentage (p < 0.05) of Listeria-positive samples than did use of either LRB or UVM alone. Silk et al. (2002) performed a comparison of growth kinetics for healthy and heat-injured L. monocytogenes in eight enrichment media, and LRB was shown to support higher maximum growth and faster mean repair times when compared with buffered Listeria, enrichment broth (BLEB), (M52) and UVM media (used in the FDA and USDA-FSIS procedures, respectively). In working with EL plates, they were found to have specificity and accuracy equal to that of the standard methods, and higher sensitivity, even when compared against dual primary enrichment using LRB to increase sensitivity and promote recovery of injured Listeria. The EL plate required less time to achieve the results. Further, because this system did not use enrichment, there was less risk of introducing contamination into the lab, thus making this method a viable alternative for in-house testing.