Detection and characterisation of Listeria isolates collected along the Gorgonzola PDO production chain

Considering that Listeria monocytogenes represents a concern in the production of Gorgonzola PDO cheese, this study was aimed to investigate the presence of Listeria along one Gorgonzola PDO production chain. L. monocytogenes subtype diversity was also investigated in order to identify possible reservoirs and dissemination routes. One dairy plant and its conferring farms (N=20) were selected and samples were collected in 4 visits in order to investigate seasonal variability. In each of them, producer tank milk (N=80), Gorgonzola producer tanks (N=12), Gorgonzola producer environmental samples (N=108) were collected, for a total of 200 samples. L. innocua was isolated from 3 milk samples (4%); L. monocytogenes alone was detected in 3 environmental samples (16%) and L. innocua in 11 (58%). Both species were present in 5 samples. All Listeria strains isolated were typed through ERIC and REP PCRs. For each samples, if possible 5 colonies were selected for typing, for a total of 76 isolates. The analyses allowed the differentiation of L. monocytogenes and L. innocua in 9 and 7 PCR profiles (P), respectively. The former showed an overall similarity value of 75% and the latter of 81%. P1 (N=3) of L. monocytogenes was present in multiple sources (ripening surfaces and salting equipment) and P5 (N=6) in different sampling rounds (January and June 2009). P1 (N=25) and P2 (N=15) of L. innocua were found in multiple sources (performing equipment, ripening surfaces, salting equipment, moving carts) and in multiple sample rounds (P1: May 2008, January and June 2009; P2: June and October 2009). The presence of a few highly similar strains of L. monocytogenes in the production chain, as suggested by the profiles retrieved from different sources/sampling rounds, indicates the presence of persistent and niche-adapted strains. However, no profile was shared between raw milk and environmental samples, suggesting that the plant environment, not the incoming raw milk, is the source of strains contaminating Gorgonzola cheese.