Prevalence and molecular characterization of Listeria monocytogenes in two Sardinian shellfish depuration facilities

The aim of the present study was to evaluate the occurrence of Listeria monocytogenes in shellfish (Mytilus galloprovincialis and Ruditapes decussatus) collected before and after depuration from two purification centres (A and B) located in Sardinia (Italy). The detection and enumeration of L. monocytogenes were carried out according to the international standard methods ISO 11290-1:1996/2004 and 11290-2:1998/2004. A total of 35 L. monocytogenes isolates were obtained and investigated for the presence of virulence-associated genes using PCR assays. The prevalence of L. monocytogenes in samples of M. galloprovincialis and R. decussatus collected at the harvesting time (T0) was 37% and 70% respectively. The prevalence of L. monocytogenes in samples collected after purification (T1) was 30% (M. galloprovincialis) and 60% (R. decussatus). None of positive samples presented L. monocytogenes at counts higher than 100 CFU/g. No significant associations were observed between the occurrence of L. monocytogenes in shellfish and environmental parameters of water. Although illnesses due do L. monocytogenes have so far only been linked to the consumption of smoked shellfish, the occurrence of L. monocytogenes strains harbouring virulence-associated genes should be considered a potential risk for consumers.