Cluster of human listeriosis cases traced by molecular typing to taleggio cheese, Italy

Objectives: Listeria monocytogenes is a foodborne pathogen responsible for severe invasive disease in humans and animals. A number of large outbreaks have been described in many industrialised countries, but most cases are considered to be sporadic. Frequent clustering was detected by a retrospective subtyping analysis of 24 human isolates identified from apparently sporadic cases of infection in the Lombardy region, Italy, in the years 2006–2007, by using serotyping and DNA-based subtyping methods. The objective of this study was to evaluate a possible causative role of L. monocytogenes strains isolated from cheese and salami samples on sale in the same geographic area. Methods: Serotyping All strains tested were serotyped following the manufacturer's instructions using commercial specific antisera (Denka Seiken, Tokyo, Japan). Ribotyping Isolates were characterised by automated ribotyping using the RiboPrinter® (Qualicon, Inc. Wilmington, DE, USA) and selecting EcoRI as the restriction enzyme. Ribotypes were firstly classified into ribotypes by automatic classification and, then, some isolates were submitted to manual classification after visual inspection. PFGE analysis PFGE was performed according to PulseNet protocol with enzymes AscI and ApaI. New PFGE profiles obtained by AscI were marked by subsequent numbers. Closely related patterns, differing from each other by one to three bands, were assigned an additional capital letter. Indistinguishable or closely related strains were subsequently cleaved with ApaI. Results: Seven clusters including two to three isolates were identified within human isolates. Isolates from salami were heterogeneous but all were different by ribotyping and PFGE from the available human isolates. Taleggio and gorgonzola cheese samples proved to contain two different strains of L. monocytogenes. A cluster, including human isolates from the geographic area under study, consisted of three strains indistinguishable by serotyping – serotype 1/2b, PFGE after AscI and ApaI digestion and EcoRI ribotyping – ribotype DUP-1034 – from L. monocytogenes strains detected on the rinds of Taleggio cheeses produced in an Italian plant. Conclusion: Listeriosis control strategies should include subtyping of human isolates and take into account that a large proportion of cases may represent foodborne outbreaks. PFGE and ribotyping databases representing isolates from different sources may be invaluable to identify source-specific subtypes.