Diversity and persistence of Listeria monocytogenes within the Gorgonzola PDO production chain and comparison with clinical isolates from the same area

Listeria monocytogenes causes invasive syndromes with high fatality rates in specific population groups. Cheeses have been commonly implicated in outbreaks worldwide. Gorgonzola is a cheese only produced in Northwestern Italy (it is the third Italian cheese in terms of production and export) and L. monocytogenes is frequently isolated from the production chain. The aims of this study were to assess the distribution of L. monocytogenes Virulence Types (VTs) in isolates collected in Gorgonzola processing plants and to determine the presence of Epidemic Clones (ECs). Fifty-Six L. monocytogenes strains collected between 2004 and 2016 from cheese and environmental samples were subtyped with Multi-Virulence-Locus Sequence Typing (MVLST) and compared to previously typed strains. Most isolates (n = 50) belonged to two new VTs (VT113 and VT114). The remaining isolates belonged to previously identified VTs: VT14-ECVIII (milk chocolate outbreak, 1994, USA) and VT80 (ricotta salata outbreak, 2012, USA). VT14, VT80 and VT113 were shared with isolates from apparently sporadic human cases in the same geographical area and temporal period (Piedmont and Lombardy, 2005–2016). The overall L. monocytogenes population appears to be homogeneous and may be characteristic of Gorgonzola production. Nevertheless, the detection in cheese and environmental samples of VTs observed in clinical isolates or outbreak related strains (VT80, VT14) contributed to better describe the current scenario and pointed out the need for increased surveillance.