Controlling Listeria through Manufacturer Intervention

The International Commission on Microbiological Specifications for Foods (2002) provides guidance on environmental sampling sites and zones, which are ranked according to risk for product contamination. In addition to environmental monitoring, establish a corrective action plan that functions to correct the situation when the prevalence of environmental contamination is beyond acceptable limits.

Standard methods for detection of Listeria from environmental sampling can take five days or more for a confirmed positive result (USDA/Food Safety Inspection Services [FSIS]; Microbiology Laboratory Guidebook [MLG] 8.04, 2005; Hitchins; 2002; ISO 11290).

Fortunately, an environmental Listeria (EL) plate provides a rapid and cost-effective tool for food manufacturers to improve the quantity and quality of data received to better detect, manage and prevent Listeria in their plant environment. Rapid results are provided in 29+/- 2 hours (including a one-hour repair step) from the time of sample collection.

This EL plate has a number of advantages. For one, it allows labs to test more often. Compared to real-time PCR ELISA-based and other rapid methods that require use of costly equipment, EL Plates are extremely cost-effective. Compared with traditional protocols, which require preparation of enrichment and plating media, the EL Plates are ready-made, allowing more testing and less media preparation time. This format is particularly suitable for small to very small food manufacturers who are limited by lab size, lack of incubator space, and lack of analytical equipment.

Another advantage is that results can be interpreted quantitatively, providing more information to identify hot spots, niches of resident strains and contamination sources for appropriate corrective action. Routine environmental monitoring and testing through use of Listeria as indicator organisms for the pathogen L. monocytogenes allows the establishment of a baseline, which can be used for comparative purposes, to observe trends or to detect a contamination problem. Once plant history and normal values have been established for a particular plant, threshold limits can be established and used to determine when corrective action is required. Online sampling during production, coupled with use of quantitative data, can indicate times when periodic cleaning and sanitation are necessary to reduce numbers and overall probability of product contamination.

The EL Plate protocol also provides a one-hour period of nonselective enrichment in buffered peptone water (BPW) to resuscitate potentially stressed Listeria, which may exist as a result of exposure to chemical sanitizers, heat, frozen sampling sites or dry/starvation environments. Failure to resuscitate injured Listeria may result in these organisms escaping detection if highly selective enrichment media are used for recovery (Donnelly, 2002). Injured Listeria can repair damage, go on to regain pathogenicity and grow to high levels in foods.

During routine microbiological analysis, it is essential to account for all Listeria, both healthy and injured. Failure to detect injured organisms may produce misleading negative results and provide a false sense of security with respect to risks that the presence of Listeria poses in the food processing environment. In addition, the EL Plate format requires no enrichment and is sealed by a protective film so it presents less cross-contamination risks. Many small companies routinely perform traditional indicator tests such as coliform counts to indicate the potential presence of E. coli, or standard plate counts to provide an indication of overall bacterial numbers. Companies elect these tests instead of specific pathogen testing because of concerns pathogens present in the laboratory environment. However, coliform media and standard plate count agar can support the growth of pathogens, and all growth media must be treated to prevent the spread of pathogenic organisms.

Regarding specific pathogen testing for L. monocytogenes, traditional regulatory protocols, such as those described by the USDA-FSIS, require use of primary and secondary liquid enrichment media prior to plating on selective media such as modified oxford agar (MOX). In liquid enrichment media, Listeria, if present in a sponge or other environmental sample, can grow to high numbers. During transfers, the potential exists for cross-contamination of the laboratory, which could lead to inadvertent plant contamination. Further, this format is ideal for shipment of suspect plates requiring follow-up to consulting laboratories, where sub-typing to identify potential for resident subtypes can be performed. EL plates can be easily coded for sample identification, and the sealed film format provides confidence that such transfer could be managed without concerns for cross-contamination or sample mix-up. The small size and lack of bulk of the EL plates means that many samples can be efficiently and cost-effectively shipped for follow-up confirmation and analysis.