Listeria monocytogenes in milk and cheese: studies of prevalence, behaviour and growth modelling during cheesemaking and shelf life

Among food-borne pathogens, L. monocytogenes represents one of the most serious food safety concerns. In particular, dairy products are often a source of this infection. During 2007 and 2009 in Italy there was an increase of notifications of listeriosis with the most cases reported in the Centre-North of Italy. This is probably attributable both to a real increase of listeriosis in Italy and to surveillance implementation. However, statistically significant increasing trends in listeriosis notification rates from 2005 to 2009 were noted in Italy as elsewhere in Europe (EFSA/ECDC, 2011). In the present research studies, raw milk, raw milk cheeses and Gorgonzola cheese (frequently contaminated dairy products) were considered like to: a) monitor the prevalence of L. monocytogenes: the collected data may be a useful tool for the quantitative risk assessment study for human listeriosis linked to the consumption of raw milk and cheese made from raw milk or Gorgonzola cheese in Italy; b) study the behaviour of L. monocytogenes in cheeses as a function of the environmental condition, when contaminated at different time points (in milk, during the cheesemaking, during the ripening or the storage) to investigate the effect of physicochemical and microbiological changes on the behaviour of L. monocytogenes during the manufacture process. These informations may increase knowledge concerning the impact of the process on the survival of pathogenic microorganisms. develop useful tools to predict the behaviour of the pathogen as a function of the environmental conditions (tempertaure, pH, aw, lactic acid bacteria competition etc.): these may be considered valuable tools to support the monitoring surveys carried out by officers of the Regional Veterinary Authority, especially when the products (milk or cheeses) are found positive to the presence of L. monocytogenes. In this case, knowing the product storage conditions (storage temperature profile and duration of shelf life) allows to predict the growth of L. monocytogenes, as to assess the safety of the product before it reaches the final consumer.