Structure-activity relationships and antibacterial membrane disruption of monoterpenoid ketones and lactones

As essential flavoring substances in the food industry, γ/δ-lactones and chiral monoterpenoid ketones are known for broad-spectrum antibacterial activity, yet systematic studies on their structure-activity relationships and mechanisms remain limited. This study evaluated the antibacterial activity and mechanisms of these compounds against foodborne pathogens. D-Carvone and L-carvone exhibited the strongest activity, with minimum inhibitory concentrations (MIC) of 0.5 mg/mL and minimum bactericidal concentrations (MBC) of 1.0 mg/mL against Escherichia coli and Staphylococcus aureus. Growth curve analysis revealed that chiral ketones bearing an α,β-unsaturated carbonyl moiety exhibited superior inhibitory effects over saturated analogs and lactones. Mechanistic studies demonstrated that these active compounds primarily disrupted the bacterial cell membrane, as evidenced by increased extracellular conductivity, leakage of proteins and nucleic acids, dissipation of membrane potential, and irreversible morphological alterations including cell shrinkage, membrane collapse, and pore formation observed by scanning electron microscopy (SEM). Molecular docking showed D- and L-carvone had the highest binding affinities to target proteins, correlating with their lowest MICs. Both the electrophilic α,β-unsaturated carbonyl and hydrophobic monoterpene skeleton likely drive membrane disruption. Time-resolved assays revealed slightly faster kinetics for D-carvone, though without affecting MICs. Overall, both enantiomers exhibit potent in vitro antibacterial activity via membrane disruption, warranting further evaluation in food systems.