Environmental and Finished Dairy Product Sampling Monitoring for Listeria monocytogenes
1.3 Background

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Environmental sampling is an important verification tool for assessing trends as well as the source, and dispersion of any microbiological contamination in food establishments. One microbial pathogen of concern in the food industry is Listeria monocytogenes (Lm). Listeria spp. is widely distributed in nature and hence, could be present in dairy processing environments and lead to a serious concern for the dairy industry. Presence of Lm in dairy products has resulted in outbreak situations, product recalls, destruction of contaminated product, extensive sanitation procedures and major financial losses for dairy operators. Environmental sampling and testing for Lm in the processing environment can permit earlier identification of any potential persistent contamination and provide an increased ability to identify and protect against Lm contamination of the finished products.

Listeria monocytogenes is a gram-positive, non-sporeforming facultative anaerobic rod bacteria which grows between -0.4 and 50°C. This microorganism has been isolated from plant, soil, and surface water samples, and has also been found in silage, sewage, milk of normal and mastitic cows, and human and animal feces. Listeriosis can manifest itself in two different forms, namely, invasive and non-invasive. Invasive listeriosis typically occurs in people with weakened immune systems (e.g. patients with AIDS, cancer patients, elderly, etc.), while non-invasive listeriosis can occur in anyone if large numbers of bacteria are consumed (e.g., >1000 cfu/g). The Center for Disease Control and Prevention reported that listeriosis had the second highest case fatality rate (20%) and the highest hospitalization rate (92.2%) of all infections caused by foodborne pathogens. This elevated degree of severity highlights the importance of minimizing exposure, particularly among populations at higher risk.

Although Lm is eliminated by pasteurization, post-processing contamination of dairy products is possible so control of Lm in processing plant environments is essential. Environmental testing provides an opportunity to identify problems before they lead to finished product contamination, potential health risk to consumers or product recalls.

The 2008 Canadian listeriosis outbreak linked to RTE meat products and the subsequent Investigator's Report resulted in the recommendation that Health Canada (HC) revise its 2004 "Policy on Listeria monocytogenes (Lm) in Ready to Eat (RTE) food" to update and clarify its position with respect to all high risk RTE foods. In summary, the 2011 HC policy recommends revised practices that are aimed at reducing the risk of Lm contamination in establishments as well as in products. The Listeria policy has categorized RTE foods into two categories based upon health risk. Category 1 RTE foods are those foods which can support the growth of Lm (Lm is assumed to grow in RTE foods with a pH greater than or equal to 4.4 and with a water activity (aw) greater or equal to 0.92). Category 2 contains two subgroups: 2A) RTE food products in which limited growth of Lm to levels not greater than 100 cfu/g can occur throughout the stated shelf-life and 2B) RTE food products in which the growth of Lm cannot occur throughout the stated shelf-life (foods with a pH of less than 4.4, regardless of the aw, nor in foods with aw of less than 0.92, regardless of the pH, combinations of pH and aw (e.g., pH < 5 and aw < 0.94 and frozen foods).

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