FOOD MICROBIOLOGY Food Microbiology 24 (2007) 444–451 Growth characteristics of Listeria monocytogenes, Listeria welshimeri and Listeria innocua strains in broth cultures and a sliced bologna-type product at 4 and 7 1C U. Nufer, R. Stephan à , T. Tasara Institute for Food Safety and Hygiene, Vetsuisse Faculty University of Zurich, Winterthurerstrasse 272, CH-8057 Zurich, Switzerland Received 7 May 2006; received in revised form 26 October 2006; accepted 27 October 2006 Available online 8 December 2006 Abstract The growth characteristics of meat processing plant-derived field strains of Listeria monocytogenes, L. welshimeri and L. innocua were analyzed. The strains were inoculated in BHI broth cultures and incubated at 4 and 7 1C. Growth curves were determined by colony counting for 28 days. Significant variations were detected in the growth properties of these field-derived strains. In particular some of the L. monocytogenes strains displayed better cold stress tolerance. These discrepancies in growth behavior were more apparent in the cultures at 4 1C compared to 7 1C. Similar growth characteristics were observed for selected L. monocytogenes strains also in food challenge tests based on a sliced bologna-type product. The results stress the need for more evaluation of field strain growth characteristics and incorporation of such information in relevant predictive microbial growth models for L. monocytogenes risk assessment in naturally contaminated food products. r 2006 Elsevier Ltd. All rights reserved. Keywords: Listeria spp.; Growth characteristics; 4 1C; 7 1C 1. Introduction The bacteria of the genus Listeria are widely distributed in nature as well as in different food processing and storage environments. Listeria monocytogenes as a food-borne pathogen has significant public health and economic impacts. The infection of humans may result in severe clinical disease as well as high mortality (Posfay-Barbe and Wald, 2004). Many food products have been implicated as routes for human infection (Farber and Peterkin, 1991; Rocourt et al., 2000). Furthermore L. monocytogenes contamination is one of the leading causes of recalls in industrially processed foods due to microbiological safety concerns. The Listeria spp. are psychrotolerant and have the capacity to grow from 1 to 45 1C, but their optimal growth temperatures range from 30 to 37 1C(Wilkins et al., 1972; Seeliger and Jones, 1986). The ability to proliferate at low environmental temperatures obviously presents a major challenge to food safety with regard to L. monocytogenes. These foodborne pathogens persist in food processing environments and proliferate during storage of chilled food products. One aspect of risk assessment of L. monocytogenes in food products depends on the application of predictive microbiology growth models (Soboleva et al., 2000). The generation and application of such predictive models relies on knowledge about the growth characteristics of L. monocytogenes under different environmental para- meters. Important parameters in this regard include the type of food matrix, storage temperature conditions, water phase salt/water activity and pH, as well as other food environment-related intrinsic and extrinsic factors. Along these lines several L. monocytogenes predictive growth models have been developed and evaluated (Koutsoumanis and Sofos, 2005; Lu et al., 2005; Francois et al., 2006). L. monocytogenes strain diversity with regard to the organism’s response to food processing and storage-related environment challenges is also another factor that influ- ences the predictive power of microbial growth models ARTICLE IN PRESS www.elsevier.com/locate/fm 0740-0020/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fm.2006.10.004 à Corresponding author. Tel.: +41 44 635 8651; fax: +41 44 635 8908. E-mail address: stephanr@fsafety.unizh.ch (R. Stephan).