Antimicrobial mechanism of morusin against Bacillus cereus and its potential as an antimicrobial agent

Bacillus cereus, a food-borne pathogen, threatens human health and food safety. The quest for biologically active substances from natural sources acting as antimicrobial agents for the replacement of antibiotics and chemical additives has received increasing attention. In this study, the antimicrobial mechanism of morusin against B. cereus was outlined, as well as its function as a food supplement to extend the shelf-life of skimmed milk was assessed. Our findings demonstrated that the minimum inhibitory concentration (MIC) value of morusin against B. cereus was 2.25 μg/mL, inhibiting biofilm formation and reducing intracellular ATP and membrane depolarization by disturbing the permeability of the B. cereus cytoplasmic membrane. Damage or collapse of B. cereus cell membranes produced by morusin was identified using the field gun scanning electron microscope (FEG-SEM). Transcriptomic assessment indicated that the two-component system of B. cereus (narG, narH, narI, phoP, phoR, motA) responded positively to morusin within an adverse environment. Moreover, several genes associated with cell membrane and cell wall generation (murA, murB, ddlB) and amino acid metabolism (argF, argH, BCN_RS02850) were significantly downregulated, potentially accelerating the death of B. cereus. In addition, metabolomic investigation suggested that morusin restricted pyruvate metabolism, cysteine and methionine metabolic pathways, and disturbed the intracellular amino acid metabolism of B. cereus. Ultimately, 4.5 μg/mL of morusin was determined to significantly inhibit the growth of B. cereus within skimmed milk. Overall, morusin can be utilized as a natural antimicrobial agent for regulating B. cereus contamination in skimmed milk.