L. monocytogenes in a cheese processing facility: Learning from contamination scenarios over three years of sampling I. Rückerl a,c , M. Muhterem-Uyar a,c , S. Muri-Klinger a , K.-H. Wagner c , M. Wagner a,b , B. Stessl a, ⁎ a Institute of Milk Hygiene, Milk Technology and Food Science, Department of Veterinary Public Health and Food Science, University of Veterinary Medicine, Vienna, Veterinärplatz 1, 1210 Vienna, Austria b Christian Doppler Laboratory for Molecular Food Analysis, University of Veterinary Medicine, Veterinärplatz 1, 1210 Vienna, Austria c Department of Nutritional Sciences, Faculty of Life Sciences, University of Vienna, Althanstraße 14, 1090 Vienna, Austria abstract article info Article history: Received 18 April 2014 Received in revised form 17 July 2014 Accepted 1 August 2014 Available online 7 August 2014 Keywords: Listeria monocytogenes Food processing Contamination scenario The aim of this study was to analyze the changing patterns of Listeria monocytogenes contamination in a cheese processing facility manufacturing a wide range of ready-to-eat products. Characterization of L. monocytogenes isolates included genotyping by pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST). Disinfectant-susceptibility tests and the assessment of L. monocytogenes survival in fresh cheese were also conducted. During the sampling period between 2010 and 2013, a total of 1284 environmental samples were investigated. Overall occurrence rates of Listeria spp. and L. monocytogenes were 21.9% and 19.5%, respec- tively. Identical L. monocytogenes genotypes were found in the food processing environment (FPE), raw materials and in products. Interventions after the sampling events changed contamination scenarios substantially. The high diversity of globally, widely distributed L. monocytogenes genotypes was reduced by identifying the major sources of contamination. Although susceptible to a broad range of disinfectants and cleaners, one dominant L. monocytogenes sequence type (ST) 5 could not be eradicated from drains and floors. Significantly, intense hu- midity and steam could be observed in all rooms and water residues were visible on floors due to increased cleaning strategies. This could explain the high L. monocytogenes contamination of the FPE (drains, shoes and floors) throughout the study (15.8%). The outcome of a challenge experiment in fresh cheese showed that L. monocytogenes could survive after 14 days of storage at insufficient cooling temperatures (8 and 16 °C). All efforts to reduce L. monocytogenes environmental contamination eventually led to a transition from dynamic to stable contamination scenarios. Consequently, implementation of systematic environmental monitoring via in- house systems should either aim for total avoidance of FPE colonization, or emphasize a first reduction of L. monocytogenes to sites where contamination of the processed product is unlikely. Drying of surfaces after cleaning is highly recommended to facilitate the L. monocytogenes eradication. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Human listeriosis is a severe food-borne disease caused by Listeria monocytogenes, a Gram-positive environmental saprophyte, that is capable of switching into a facultative intracellular pathogen (Freitag et al., 2009). The main route of transmission to susceptible individuals, such as pregnant women, the elderly and immunocompromised pa- tients, is contaminated food (McLaughlin et al., 2004). Over the last eight years, incidence rates ranging from 0.3 to 1.3 per 100,000 capita have been reported in European countries, the U.S., Canada and Australia (Todd and Notermans, 2011). Cheese-related foodborne out- breaks were often linked to major deficiencies in good hygiene practices, inadequate process monitoring and fluctuation in L. monocytogenes con- tamination dynamics (Fretz et al., 2010; Koch et al., 2010; Jackson et al., 2011; Schoder et al., 2012; Yde et al., 2012). L. monocytogenes is difficult to eradicate due to surface and niche colonization, resistance conditions of low pH (b 4.4), water activity (b 0.94) and growth at refrigeration tem- peratures (Ortiz et al., 2010; Tasara and Stephan, 2006). Recently an in- crease in the prevalence of L. monocytogenes serotype 1/2a has been observed among food and human sources (Gilmour et al., 2010; Lambertz et al., 2013; Lomonaco et al., 2013; Mammina et al., 2013). Al- though Listeria spp. are generally sourced to soil environments, carriage by wild and domesticated animals, and humans, specific routes of entry to food processing facilities are still not fully understood. Even low grade contamination (b 10 cfu/g) of raw materials with L. monocytogenes poses a major challenge to the food industry, with International Journal of Food Microbiology 189 (2014) 98–105 ⁎ Corresponding author at: Institute of Milk Hygiene, Milk Technology and Food Science, Department of Veterinary Public Health and Food Science, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210 Vienna, Austria. Tel.: +43 1 25077 3502; fax: +43 1 25077 3590. E-mail address: Beatrix.Stessl@vetmeduni.ac.at (B. Stessl). http://dx.doi.org/10.1016/j.ijfoodmicro.2014.08.001 0168-1605/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect International Journal of Food Microbiology journal homepage: www.elsevier.com/locate/ijfoodmicro