Characterization of bacteriocins production and safety properties in non starter Enterococcus faecium strains isolated from home-made white brine cheese.

In last few decades, the increasing consumer demand for natural and chemical additive-free products has stimulated food industry to look for novel and alternative strategies for food biopreservation. In such a perspective, a promising role could be played by lactic acid bacteria (LAB) producing bacteriocins, defined as ribosomally synthesized antimicrobial peptides that exhibit antagonism mainly against Gram-positive bacteria. Among LAB, Enterococcus spp. strains are highly adapted to several food systems because of their strong tolerance to salts and acids. They are often found in high numbers and are believed to contribute to cheese ripening and to the development of aroma, especially in cheese products made in the Mediterranean area. Enterococcus spp. species have been previously reported to produce bacteriocins, with few strains belonging to E. faecium and E. faecalis being investigated also with regard to their potential as probiotics. However, such a role in Enterococcus spp. is still controversial considering both their increased association with nosocomial infections and their harbouring multiple antibiotic-resistant genes. In addition, several putative virulence factors have been recently described in enterococci. Therefore, any bacteriogenic Enterococcus spp. strain, and, generally, every bacteriocin producing isolate, should be carefully evaluated also for safety and virulence traits. In this work, four LAB strains, isolated from home made white brine cheese, were selected for their effective inhibition against Listeria monocytogenes. According to their biochemical and physiological characteristics, the strains were classified as members of Enterococcus genus, and then identified as E. faecium by 16S rDNA sequecing. Their bacteriocin production and inhibitory spectrum were characterized together with the occurrence of several bacteriocin genes (entA, entB, ent P, ent L50B). Their virulence potential and safety were assessed both using PCR targeted to the genes gelE, hyl, asa1, esp, cylA, efaA, ace, vanA, vanB, hdc1, hdc2, tdc and odc and by phenotypical tests for antibiotic resistance, gelatinase, lipase, DNAse and α- and β-haemolysis. The E. faecium strains harboured at least one enterocin gene while the occurrence of virulence, antibiotic resistance and biogenic amines genes was limited. Considering their strong antimicrobial activity against L. monocytogenes strains, the four E. faecium strains exhibited promising potential as bio-preservatives cultures for fermented food productions. Moreover, on the basis of their technological properties, the results of this study suggest that they could be efficiently used as adjunct non-starter lactic acid bacteria (NSLAB) in many milk processing applications.