Listeria monocytogenes contamination sources in sheep cheese processing plants and strains virulence genes typing

A survey on Listeria monocytogenes (Lm) contamination patterns was carried out in 10 sheep cheese processing plants, located in Sardinia (Italy). The samples were collected from the processing line during production. The samples were taken in cheese and ricotta processing area from food contact surfaces (raw milk filters, moulds, tables, ripening shelves and cheese and ricotta washing machine) and non food contact surfaces (floor drains or floors/walls). Pooled samples from rind surfaces of hard cheese and ripened ricotta were also taken. Lm strains were isolated from 48 samples (18.8%) taken from 9 cheese factories (90%) and other Listeria spp strains were isolated from 39 samples (15.3%) from 10 cheese factories (100%). Lm was detected in raw milk filters (16.7%), cheese rinds (5.4%), ripened ricotta surfaces (18.2%), cheese moulds (20%), tables and shelves (9.8%), cheese washing machines (60%), ricotta washing machines (20%), drains or walls/floors surfaces (19.4%). Lm strains analysed for serotype with traditional and molecular method belonged to groups 1/2a (62,8%), 4/b (19,8%), 1/2b (14,0%) and 1/2c (3,5%). All Lm strains analysed for virulence genes prfa, clp, inlA, inlB, inlC, hly, lisR, acta A , acta A were found as positive.