Estimation of Listeria monocytogenes transfer coefficients and efficacy of bacterial removal through cleaning and sanitation
Highlights
► Listeria monocytogenes transfer coefficients (TCs) and sanitization were analyzed. ► TCs were generally low, indicating that contamination can be spread to many food items. ► Sanitizing efficacy was strongly affected by external conditions. ► Strict adherence to cleaning and sanitizing SOPs is thus crucial to assure efficacy. ► Stochastic models of transfer and sanitization were derived for risk assessments.
Introduction
Listeria monocytogenes has been estimated to cause approximately 1600 human invasive listeriosis cases in the United States each year (Scallan et al., 2011). Of these, as many as 1455 cases annually are predicted to result in hospitalization and 255 cases are expected to result in death, making L. monocytogenes one of the leading causes of death due to foodborne illness in the United States (Scallan et al., 2011). Epidemiological studies of sporadic cases, food surveys, and outbreak investigations have provided compelling evidence for contaminated ready-to-eat (RTE) foods as principal sources of listeriosis (FAO/WHO, 2004, FDA/FSIS, 2003). A risk assessment of 23 selected categories of RTE foods, published in 2003, identified deli meats as the food category with the greatest associated public health risk (FDA/FSIS, 2003). Importantly, deli meat products sliced and packaged at retail are more frequently contaminated, and with higher bacterial concentrations, compared to those sliced and packaged by the manufacturer at the processing facility (Endrikat et al., 2010, Gombas et al., 2003). Risk assessments estimate that more than 70% of those listeriosis cases attributable to deli meats are associated with deli meats sliced at retail and formulated without growth inhibitors (Endrikat et al., 2010, Pradhan et al., 2010). Cross-contamination at retail, combined with bacterial growth during storage, therefore likely represents a fundament risk factor for listeriosis. Environmental surfaces in retail operations can be frequently contaminated with L. monocytogenes (Hoelzer et al., 2011, Sauders et al., 2009). For instance, in a survey of retail operations in the state of New York, environmental contamination with L. monocytogenes was detected in 60% of the establishments (Hoelzer et al., 2011). Notably, 3% (5/183) of slicers and 4% (13/314) of other utensils in these establishments were contaminated with L. monocytogenes, emphasizing the considerable cross-contamination potential.
The factors contributing to L. monocytogenes cross-contamination at retail, however, are currently not well understood. Mathematical models of environmental cross-contamination offer a valuable alternative to observational studies in retail facilities or intervention studies in mock delis as they allow for the expeditious and cost-effective evaluation of cross-contamination risks and enable exploration of the effects of different management practices. However, data to populate such models are not readily available. Laboratory studies have analyzed bacterial transfers between solid surfaces and food, with sometimes contradictory results (see Perez-Rodriguez et al., 2008 for a review). Several factors, including the biological state of the bacterium (Midelet and Carpentier, 2002), the nature and intensity of the contact (Chen et al., 2001), the surface structure of the contamination source (Midelet and Carpentier, 2002, Silva et al., 2008), and the food type (Vorst et al., 2006) seem to impact transfer dynamics. It has been shown that a contaminated slicer blade can spread L. monocytogenes to many uncontaminated food slices, but the transfer dynamics vary considerably with the physico-chemical composition of the sliced product (Keskinen et al., 2008a, Sheen, 2008, Sheen and Hwang, 2010, Vorst et al., 2006). Cleaning and sanitation may effectively reduce bacterial contamination in the environment and therefore lower contamination of food, but the efficacy of different sanitizers in reducing L. monocytogenes contamination is highly dependent on a number of extrinsic factors related for instance to pathogen type, ambient temperature, water hardness, surface structure, or the presence of protein residues (Marriott and Gravani, 2006, New York State Department of Agriculture and Markets, 2011).
Here we synthesize the relevant peer-reviewed scientific literature to derive probability distributions that model i) transfers between various surfaces and food; ii) transfer dynamics during slicing of foods; and iii) removal of bacteria through cleaning and sanitizing. In a second step we derive mathematical models that can be used to describe these processes. The derived probability distributions and models can be incorporated into comprehensive models of L. monocytogenes cross-contamination to evaluate the public health impact of environmental contamination with L. monocytogenes in places where RTE foods are handled.
Section snippets
Materials and methods
The relevant peer-reviewed scientific literature was identified through searches in the National Center for Biotechnology Information (NCBI) PubMed database, cross references in related published manuscripts, and auxiliary data sources such as the Google search engine. Literature searches for transfer coefficients (including those specific to slicers) and for cleaning and sanitizing were performed in June 2009 and December 2010, respectively. For transfer coefficient studies (including those
Results
A total of 37 peer-reviewed studies met our inclusion criteria, and 2308 data points were extracted from these studies.
Discussion
Several food-safety related studies of bacterial transfer coefficients and cleaning and sanitization efficacy have been published in recent years. Unfortunately, the study results summarized here appear highly variable and conclusions sometimes seem contradictory. It has largely remained unclear whether this high inter-study variability reflects experimental artifacts such as experimental design differences or disparate detection and enumeration methods, or whether it reflects true biological
Conclusion
In general, transfer coefficients appear to be low, indicating the potential for wide-spread cross-contamination at low concentrations. Sanitizing can efficiently remove contamination under optimal conditions, but the true efficacy of bacterial inactivation in practice remains unclear given the large number of factors that may significantly hamper efficacy. The current probabilistic models derived here allow for the expeditious and cost-effective evaluation of different cross-contamination
Acknowledgments
This work was supported in part by appointments to the Research Participation Program at the Center for Food Safety and Applied Nutrition administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the US Department of Energy and the U.S. Food and Drug Administration. Support for this work was also provided by Virginia Tech in collaboration with the U.S. Department of Agriculture, Food Safety and Inspection Service (FSIS contract # AG-3A94-P-08-0166
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