Evolution of Listeria monocytogenes populations during the ripening of naturally contaminated raw ewe’s milk cheese Noe ´mia Gameiro a , Suzana Ferreira-Dias b , Mass Ferreira a , Luisa Brito a, * a Laborato ´ rio de Microbiologia, DBEB, Instituto Superior de Agronomia, Tapada da Ajuda 1349-017 Lisboa, Portugal b Centro de Estudos Agro-Alimentares, DAIAT, Instituto Superior de Agronomia, Tapada da Ajuda 1349-017 Lisboa, Portugal Received 25 April 2006; received in revised form 1 August 2006; accepted 7 August 2006 Abstract The aim of this work was to study, in loco, the evolution of Listeria monocytogenes populations, during ripening (7, 42, 60 and 120 days) of naturally contaminated raw ewe’s milk cheese. Two batches of cheese consisting of 20 or 16 cheeses were obtained from two farmstead cheesemakers, respectively. A significant increase in numbers of L. monocytogenes was observed for both batches, from 7 to 42 days of ripening. These results suggest that this type of cheese has potential to support the survival of L. monocytogenes, while stressing the importance of cheese contamination in the dairies by resident strains. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Listeria monocytogenes; Cheese ripening; Environmental contamination 1. Introduction Listeria monocytogenes is a foodborne bacterium recog- nised as pathogenic for both humans and animals. Because of its versatility, Listeria is able to persist in the food indus- try environment, for several years (Unnerstad et al., 1996), probably in a biofilm state. Post-processing contamination of food with L. monocytogenes is a critical problem of pub- lic health. Several outbreaks of listeriosis were linked with the consumption of minimally processed and ready to eat (RTE) foods (Aureli et al., 2000; Brett, Short, & McLauch- lin, 1998). These reports highlighted the importance of cross-contamination of processed foods from environmen- tal sources. Cheese is one RTE type of food that has been associated with foodborne listeriosis (Donnelly, 2001). Lis- teria is widely disseminated in the rural environment and consequently cheese may be contaminated at any stage from farm to table. Presently in Portugal, there is no sur- veillance for L. monocytogenes infections and consequently, there is no reported human listeriosis associated with food consumption. The type of cheese analysed in this study is a traditional Portuguese cheese made from raw ewe’s milk coagulated, at 28–30 °C, with added salt (NaCl) and plant rennet (cardoon flower), without deliberate addition of any starter culture. The curd is manually worked, molded and pressed at room temperature (20 °C) and finally is bandaged with a strap of clean cotton cloth. Attached to the side of each cheese is a casein label (Passport Ò casein marks, DSM Food Special- ties, Delft, Netherland) containing a number that identifies the daily production. The cheeses are taken to the first mat- uration room (6–10 °C) where ripening is carried out on wooden shelves for 15–21 days. At the end of this first stage of ripening, the cheeses are moved to the second maturation room (10–14 °C) where they are kept also on wooden shelves until the end (30–120 days). From the first day of production, cheeses are turned upside down daily. Usually after one week a viscous smear spontaneously appears on the surface of the cheese. Cheeses are then washed with water, generally with brushes, and bandaged again with 0956-7135/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2006.08.002 * Corresponding author. Tel.: +351 21 365 3240/351 21 365 3435; fax: + 351 21 365 3238. E-mail address: lbrito@isa.utl.pt (L. Brito). www.elsevier.com/locate/foodcont Food Control 18 (2007) 1258–1262