Microtiter plate-based assay for screening antimicrobial activity of melanoidins against E. coli and S. aureus

A rapid plate reader based method examining the antimicrobial activity of both model and food melanoidins (coffee, beer, sweet wine) is described. Antimicrobial activity against Escherichia coli and Staphylococcus aureus is evaluated as area under the growth curve compared to a control. Method was settled for an aqueous melanoidin concentration of 2 mg/ml inoculated to 106 cfu/ml culture. All tested model and food melanoidins exerted antimicrobial activity in some extent, but inhibition was significantly higher over S. aureus (Gram-positive) than E. coli (Gram-negative). Antimicrobial activity can be further quantified by expressing it as OTEV (oxytetracyclin equivalent value, μg/l) which could serve to compare the results obtained within different laboratories, methodologies and/or compounds. Results indicate that both strains have different sensitivity against the presence of melanoidins and probably different mechanism of inhibition. Procedure can be used for a rapid screening of the potential antimicrobial properties of melanoidins, and subsequently to Maillard reaction products as well, against pathogenic strains in order to isolated substances with biological activity.