The 2011 ASHS Annual Conference

Lactic acid bacteria (LAB) produce a variety of compounds with antimicrobial activity including acids, alcohols, diacetyl, hydrogen peroxide and bacteriocins. Bacteriocins have attracted many interests due to their potential use as safe and natural food preservatives and antimicrobials for control of spoilage and pathogenic microorganisms. The objective of this study were to isolate and characterize bacteriocinogenic LAB strains, test their antimicrobial effects on common spoilage and pathogenic microorganisms and further investigate selected LAB for use as bio-preservatives on fresh-cut vegetables. Fourteen different horticultural commodities and 11 different dairy products were used to isolate LAB. The antimicrobial properties of cell-free supernatants from LAB isolates were determined using the agar diffusion bioassay with Lactobacillus sakei and Listeria innocua as indicator organisms. The presence of bacteriocins was indicated by inhibition zones following the neutralization of pH and elimination of H₂O₂, and further confirmed by the addition of proteolytic enzymes. The 28 bacteriocin-producing LAB were identified by 16S rRNA gene sequencing. Molecular weights of bacteriocins were determined using SDS-PAGE method, AU values of bacteriocins and optimal growth conditions for producing bacteriocins were also investigated. In addition, selected bacteriocins were identified using LC/MS. Antimicrobial effectiveness of LAB and bacteriocins were tested against 6 different bacteria, and 3 different fungi using agar diffusion bioassay and microtiter plate method, respectively. Results indicated that bacteriocins produced by the LAB showed significant antimicrobial effects against L. innocua while acids and H₂O₂ had strong antimicrobial effects against other tested microorganisms. Challenge tests were conducted on fresh-cut salads and onions inoculated with L. innocua. The effects of LAB on control of natural microflora of fresh-cut vegetables and L. innocua were investigated. Results showed that bacteriocins producing LAB significantly reduced Listeria sp. counts (p=0.005) throughout storage at 5°C. The addition of LAB was also able to reduce the counts of yeasts and molds (p=0.011), Pseudomonas sp. (p=0.01) and total coliforms (p=0.01) compared to the controls. Therefore, LAB and/or bacteriocins have significance for use as bio-preservatives and antimicrobial agents.