Characteristics of Listeria monocytogenes isolated from tonsils of slaughtered fattening pigs in Switzerland

Of 504 tonsil samples from slaughtered fattening pigs, 5.6 % were positive for Listeria monocytogenes by culture after enrichment. The 28 L. monocytogenes isolates were analyzed to gain insights into genetic relationships and virulence-associated traits. Of the 28 isolates, 57 % belonged to serotype 1/2a (genetic lineage II), 25 % to serotype 4b (genetic lineage I) and 18 % to serotype 1/2b (genetic lineage I). These serotypes are commonly associated with human listeriosis cases. Multilocus sequence typing assigned the 28 isolates to 16 clonal complexes (CCs) and three singletons, including one new sequence type (ST768). Several of these CCs were also found in strains from human infections. Sequence analysis of the whole internalin A gene (inlA) showed that all but one isolate (CC6/serotype 4b) encoded full-length proteins. Clinical strains from human patients commonly harbor full-length inlA. On the other hand, genes for benzalkonium chloride tolerance were not found and all but one isolate lacked genes for the stress survival islet (SSI-1). Thus, tonsils of slaughtered fattening pigs can be colonized with L. monocytogenes of public health impact. To counteract this threat during slaughter, prevention of contamination of carcasses and the environment is of major importance, in particular adherence to good slaughter hygiene practice. With regard to pig tonsils, special attention must be given to the handling and contamination of head meat and pig tongues