Evaluation ofListeria monocytogenescontamination in sheep’s milk cheese-making plants

The general aim of the thesis was to evaluate the patternListeria monocytogenescontamination in sheep’s milk industrial cheese-making plants operating in the regional territory of Sardinian (Italy). Chapter 3 describes a longitudinal study conducted in 2 cheese-making plants over one year period. The objective of the study was to identify sources, sites of persistence and route of contamination within premises. Contamination mostly occurred in salting, product washing, packaging, ricotta salata storage and cheese ripening areas. The greater persistence of contamination over time was observed in washing, salting and cheese ripening areas. In Chapter 4 is presented a cross-sectional study onListeriaspp environmental contamination in 13 sheep cheese-making plants. The objective was to investigate the genetic diversity (population study) and the route of contamination (traceability) ofL. monocytogenesstrains. Strains originating from the processing environment were disseminated to other sites and food samples within and among facilities. In chapter 5 the potential pathogenicity of environmental and food isolates collected in cheese-making plants was studied by DNA sequencing. In particular the modification in nucleotide sequence of virulence factors hly, InlA and InlJ was investigated.L. monocytogenesenvironmental strains showed 90-100% of identity with reference sequences, suggesting their potential pathogenicity.