Brito, Maria Luísa de CastroSilva, Ana Carla Matos2010-11-112010-11-112010http://hdl.handle.net/10400.5/2459Mestrado em Engenharia Alimentar - Segurança Alimentar - Instituto Superior de AgronomiaListeria monocytogenes is a bacterial foodborne pathogen able to grow at refrigeration temperatures. In this work, in order to evaluate the effect of cooling on the virulence of this bacterium, under conditions of absence of growth, the effect of storage time at 7 ºC (0, 1, 7 and 30 days) in 17 isolates and two reference strains for virulence was investigated. The isolates from different sources were characterized by PFGE and their pathogenicity was assessed with plaque-forming assays in HT-29 cell monolayers. The virulence potential of the strains was expressed as the mean value of the logarithms of the number of plaques formed (log PFA). The subtyping of the isolates by PFGE showed that the isolates selected to study the effect of this cooling temperature on the virulence, showed a high genetic diversity. The results indicated a decrease in virulence after cooling in all strains, although the time dependence appeared to be a strain effect. Maintenance of virulent strains of L. monocytogenes in chilled food matrices, able to prevent its growth, may cause a significant decrease of its original virulence, and consequently a reduced risk to consumer health.porListeria monocytogenesPFGEPFAvirulencerefrigerationmaster thesis