Listeria monocytogenes prevalence, contamination levels and strains characterization throughout the Parma ham processing chain

In order to estimate prevalence, levels and patterns of Listeria monocytogenes contamination, a total of 774 swine carcasses were traced along the Parma ham production chain. Analyses were conducted on isolates originated from the same carcass, collected at different stages during processing, resulting in 0.2% (faeces at intestine removal from carcasses), 3.0% (swabbing of carcasses), 12.5% (fresh hams) and 2.0% prevalence (dry-cured hams). The highest contamination levels of L. monocytogenes were reached in fresh hams after cutting and were followed by a marked decrease during the subsequent processing stages. All the 132 isolates were characterized by serotyping and Pulsed-Field Gel Electrophoresis (PFGE). Transfer of L. monocytogenes between different stages of the processing chain was not reported, whereas processing itself has proved to be an important cause of contamination. The sole isolate of fecal origin belonged to a pulsotype that was uncommon to any of those recovered in carcasses, fresh hams and dry-cured hams, indicating that contaminations from farms does not significantly affect Parma ham production. For the majority of the strains isolated from the same production plants, PFGE profiles were highly similar. In several cases, the same pulsotypes were recurrently detected, over time, in carcasses and fresh ham samples sharing the same processing environment. Aw levels were also measured, showing that drying of the ham surface was able to induce a considerable decrease of the contamination levels, although unable to ultimately remove L. monocytogenes