Mitarbeitende

The association between hypervirulent Listeria monocytogenes and dairy products has been demonstrated recently. Smear cheese types in particular support the growth of Listeria monocytogenes in the rind. In this study, we investigated the inhibitory properties of the microflora of a PDO artisan soft smear cheese produced from thermized milk. Fresh cheeses produced in nine dairies were collected and inoculated on their surface with Listeria innocua, a surrogate of the pathogenic species. Rind samples were monitored for Listeria counts. Rind microbiomes were analyzed by amplicon sequencing and gave a total 567 ASVs assigned to four phyla, 64 genera and 106 species. In particular, Leuconostoc spp. were negatively correlated to Listeria counts throughout ripening. Inhibition of Listeria was observed in vitro for 5 out of 10 Leuconostoc mesenteroides strains isolated from the cheese rind. This inhibition was associated with plasmid encoded mesentericin Y genes in the genomes of the strains. Challenge tests were conducted with a subset of mesY positive Leuconostoc strains inoculated in the vat milk together with the starter and 0.5 cfu/ml Listeria. Efficacy of treatments was evaluated based on a qualitative detection in 25g rind at the end of shelf life. Addition of mesY positive Leuconostoc had a significant impact in situ, with 100% positive loaves for the control and 25% positive loaves for the Leuconostoc treatment. Next to the putative production of mesentericin Y, monitoring of biochemical parameters suggests that the co-metabolism of residual glucose and citrate by Leuconostoc is likely to have contributed to the protective effect.