Structural enterocin gene profiles and mode of antilisterial activity in synthetic liquid media and skim milk of autochthonous Enterococcus spp. isolates from artisan Greek Graviera and Galotyri cheeses

Abstract

The presence of eight common structural enterocin genes, singly or in varying combinations, in the genome of 15 antagonistic Enterococcus spp. previously isolated from artisan Greek Graviera and Galotyri retail cheeses was tested and associated with the mode of enterocin (Ent+) antilisterial activity of each isolate in three liquid culture media. The isolates were assigned to nine distinct strain genotypes of E. faecium (4 strains), E. durans (2) and E. faecalis (3). All strains were non-hemolytic, except for a cyl-positive E. faecalis genotype isolated from Galotyri cheese, which was strongly listericidal. All other strains varied from being listeriostatic to weakly listericidal in MRS and M17 broth, whereas all failed to inhibit listerial growth in skim milk. Two E. faecium strains retained strong Ent+ activity following neutralization and filter-sterilization of their MRS or M17 co-culture supernatants, whereas, all others required contact or proximity of their viable cells with L. monocytogenes cells in order to display activity. Additional studies to evaluate safety and potential synergistic effects of each strain genotype with starter LAB species in real milk environments will reveal the most active and truly harmless Enterococcus genotypes to be applied as co-starter or bioprotective adjunct cultures in traditional Greek cheese technologies.