Molecular Characterization of Listeria monocytogenes Strains Isolated from Italian Blue Cheese Production Plants and Comparison with Human Clinical Strains from the Same Geographical Area

Introduction: Listeria monocytogenes causes invasive syndromes with mortality rates as high as 30% in specific population groups such as elders, immuno-compromised and newborn or fetuses. Fresh and unpasteurized cheeses have been frequently implicated in outbreaks and sporadic cases both in Europe and USA. Italian blue cheese is a raw milk soft cheese mainly produced in Western Northern Italy, with a high export rate (Europe, USA, Canada, Far East and Australia) and a yearly retail turnover exceeding 500 million Euros.

Purpose: Given the above, it is important to better understand L. monocytogenes isolates obtained from blue cheese. Therefore, the aims of this study were to characterize strains of L. monocytogenes isolated in blue cheese production plants and compare them to isolates from human cases diagnosed in the same geographical area.

Methods: Forty-one L. monocytogenes strains from cheese and environmental samples obtained from 22 production plants (2004-2007) were subtyped with Multi Virulence Locus Sequence Typing (MVLST) and compared to 60 L. monocytogenes strains from sporadic human cases (2005-2014) and previously typed strains. New allelic sequences were assigned arbitrary virulence type (VT).

Results: Five VTs were identified among food/environmental samples:  83% (n = 34) showed two new allelic profiles, 12% (n = 5) were VT14 (milk chocolate outbreak, 1994, USA),  0.02% (n = 1) was VT80 (ricotta salata outbreak, 2012, USA) and 0.2% (n = 1) was VT46.  Twenty-six VTs were observed among human strains, with 62% (n = 37) of samples showing VTs previously observed in outbreaks. VT14 and VT80 were also identified in 3 and 4 of the clinical samples, respectively.

Significance: The production environment of blue cheese can allow growth and persistence of L. monocytogenes, and while this cheese shares many characteristics with other cheeses considered at risk for Listeriosis, the overall strain population appears to be homogeneous and relatively distinct from human cases strains from the same region.