Characteristics of Listeria monocytogenes strains isolated from fecal samples of healthy slaughtered livestock

Thirty-two L. monocytogenesstrains isolated from fecal samples of slaughtered pigs, cattle, and sheep were characterized to gain insights into genetic relationships and virulence-associated traits. The majority (87.5 %) of the 32 strains belonged to serotype 1/2a (genetic lineage II), followed by serotype 4b (genetic lineage I). Multilocus sequence typing (MLST) grouped the 32 strains into 10 STs that were assigned to eight clonal complexes (CC1, CC4, CC6, CC8, CC29, CC37, CC54, CC90) and two singletons (ST36, ST200). ST 709 (CC90) was designated for the first time and comprised 20 serotype 1/2a strains from cattle and sheep. PCR screening for Listeriolysin S showed the presence of this virulence factor in the four lineage I/serotype 4b strains belonging to CC1, CC4, CC6, and CC54. Sequence analysis of the whole internalin A gene (inlA) of 10 strains representing the 10 STs showed that all but one strain (CC6/serotype 4b) encoded full-length proteins. Thus, slaughtered pigs, cattle, and sheep can be asymptomatic carriers of L. monocytogenes representing a potential health risk. To encounter this threat during slaughter, prevention of contamination of carcasses and the environment is of major importance, in particular adherence to good slaughter hygiene practices. With regard to slaughtered animals, process stages with an increased risk of direct or indirect fecal contamination deserve special attention