Characterization of antimicrobial efficacy of soy isoflavones against pathogenic biofilms

•Ultrasonication technique provided 693.99 μg of isoflavones per gram of soy flour sample.•The size of isoflavone particles extracted by ultrasonication treatment ranged between 100 and 600 nm in diameter.•Listeria monocytogenes and Escherichia coli biofilms were inhibited by 10 μg/mL and 100 μg/mL soy isoflavones.•Isoflavone treatment induces cell dispersion and cell wall disruption in L. monocytogenes.

Pathogenic biofilms that form on food processing equipment/surfaces are of great concern, because these can readily lead to food spoilage, bio-fouling, food-borne illness, and their recalcitrance can result in the acquisition of multi-drug resistance. Currently available coatings do not completely inhibit microbial growth and an increased demand for such coatings means that new products will need to be developed. The unique properties of antimicrobial soy isoflavones, including their biodegradability, biocompatibility, and lack of toxicity as edible products, make their application more appealing than artificial polymer or chemical-based coatings. In this study, we evaluated the antimicrobial efficacy of soy isoflavones against pathogenic biofilms of Listeria monocytogenes, Escherichia coli, Pseudomonas aeruginosa, and Methicillin-Resistant Staphylococcus aureus (MRSA) using microtiter plate assays (MPAs), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Ultrasonication technique yielded 491 μg of isoflavones per gram of soy flour sample. MPA assays and the imaging experiments revealed that the establishment of L. monocytogenes and E. coli biofilms was inhibited by 10 μg/mL and 100 μg/mL soy isoflavones, while MRSA and P. aeruginosa were largely unaffected.