Isolation, Characterisation and Fermentation Optimisation of Bacteriocin-Producing Enterococcus faecium

PURPOSE: The aim of this work was to optimize a bacteriocin produced by Enterococcus faecium TJUQ1 and the potential application in vegetable wash solution. METHODS: The agar well diffusion method was applied to screening strains with antibacterial activity. It was identificated based on physiobiochemical characteristics and 16S rDNA sequence analysis. Response surface method (RSM) was applied to optimize bacteriocin yield and discover the best production condition. RESULTS: A total of 31 isolates of lactic acid bacteria (LAB) isolated from pickled Chinese celery were screened for bacteriocin production. Strain TJUQ1 showed an inhibition zone against Listeria monocytogenes CMCC 1595 as indicator by using agar well diffusion assay and identified as E. faecium. The culture parameters were further optimized using response surface methodology (RSM). By using Plackett–Burman (PB) design, beef extract, K₂HPO₄ and initial pH were found to be the most significant factors for bacteriocin activity. The effects of the three main factors on bacteriocin activity were further investigated using a central composite design (CCD) and the optimum composition was found to be beef extract 15.20 g/L, K₂HPO₄ 1.93 g/L and initial pH 7.19. Optimum conditions were validated by experiment in which bacteriocin activity was increased 1.78-fold (816.87 ± 5.21 AU/mL) in 18 h fermentation. The bacteriocin-containing wash solutions showed activity against L. monocytogenes CMCC 1595 inoculated onto fresh-cut iceberg lettuce within the first 3 days of storage at 4 °C. CONCLUSION: A bacteriocin-producing strain TJUQ1 isolated from pickled Chinese celery was identified as E. faecium. The strain produced a high level of bacteriocin and the bacteriocin-containing wash solutions showed activity against L. monocytogenes CMCC 1595 inoculated onto fresh-cut iceberg lettuce.