Antimicrobial activity against select food-borne pathogens by phenolic antioxidants enriched in cranberry pomace by solid-state bioprocessing using the food grade fungus Rhizopus oligosporus

Abstract

Listeria monocytogenes, Vibrio parahaemolyticus and Escherichia coli 0157:H7 are important food-borne pathogens. Control of these pathogens using synthetic antimicrobials such as currently approved antibiotics is challenging due to potential development of resistance. A profile of antimicrobials compared to a single compound could be potentially more effective. Cranberry pomace is a byproduct of the cranberry processing industry. Solid-state bioprocessing of cranberry pomace using the food grade fungus Rhizopus oligosporus improves phenolic and antioxidant profiles. We hypothesize that these phytochemicals mobilized during pomace bioprocessing could improve the antimicrobial functionality. The objective of this research was to use the food grade fungus R. oligosporus to release phenolic aglycones and relate its antioxidant functionality and diphenyl mobilization to antimicrobial activity against L. monocytogenes, V. parahaemolyticus and E. coli 0157:H7. Bioprocessing of pomace was done for 20 days with R. oligosporus. Total phenolics in water extracts were assayed using a Folin-Ciocalteu method. The antioxidant potential was measured using a β-carotene oxidation system (APF) and 1,1-diphenyl-2-picrylhydrazyl-radical (DPPH) system. Changes in phenolic profiles were analyzed using HPLC. Antimicrobial activities of the extracts during growth were tested using standard disk assays. Soluble phenolics, antioxidant activity and ellagic acid concentrations were enriched by bioprocessing and antimicrobial activity of the extracts against L. monocytogenes and V. parahaemolyticus correlated with highest soluble phenolics and APF in the same extracts. For E. coli 0157:H7 inhibition correlated with the extracts corresponding to highest DPPH and ellagic acid concentration. The bioprocessing-based antimicrobial activity depended on different phenolic functional properties of the extracts. Sensitivity towards soluble phenolics reflected in DPPH activity suggested inhibition by the disruption of the membrane by hyperacidification. Whereas, sensitivity to APF and ellagic acid suggests potential antimicrobial activity by membrane-transport disruption. The variation in sensitivity of pathogens has implications for designing new food grade antimicrobials. This bioprocessing strategy can be an innovative approach to produce broad spectrum antimicrobials against important food-borne pathogens.