Isolation and characterization of two bacteriocins produced by Enterococcus faecium strains inhibitory to Listeria monocytogenes.

A total of 4000 bacterial strains were isolated from milk, cheese and from different samples taken on the farm (silage, faeces) and screened for their antimicrobial activity against Listeria monocytogenes. Only eight of the almost 4000 strains exhibited inhibitory activity in the cell-free supernatant fluid. Two different Enterococcus strains (RZS C5 and RZS C13) were chosen for further study. Plasmid profile analysis of both revealed that RZS C5 contained a 4 kb plasmid while RZS C13 contained a 23 kb plasmid. The inhibitory compounds were proteinaceous and active against all Listeria spp. tested while most of the lactic acid bacteria were insensitive. Weak inhibition was also detected against Clostridium perfringens and some Bacillus spp. Cross-inhibition tests showed different profiles. The bacteriocins were heat-resistant and were very stable during storage, especially at low pH values. Their molecular weight was estimated at +/- 3.0 kDA. Addition of 200 AU/ml of bacteriocin resulted in an initial rapid decrease of the number of viable L. monocytogenes cells followed by slower a logarithmic decrease.