In vitro Potential Effect of Morin in the Combination with β-Lactam Antibiotics Against Methicillin-Resistant Staphylococcus aureus

Morin, a plant-derived flavonol, is known to be an effective inhibitor of Gram-positive bacteria. In this study, we explored the combined effect of morin with -lactam antibiotics against methicillin-resistant Staphylococcus aureus (MRSA), a multidrug-resistant pathogen. The anti-MRSA activity of morin was investigated by the broth microdilution method, checkerboard dilution test, and time-kill curve assay. The expression of the resistant protein, penicillin-binding protein (PBP2a) encoded by mecA, was analyzed by the Western blotting method in the presence of morin and oxacillin. An increased susceptibility of MRSA toward oxacillin was observed in the presence of morin. The protein level of PBP2a was reduced when MRSA (ATCC 33591) was treated with the combination of morin and oxacillin, indicating that the combination of morin and oxacillin potentiates the killing effect against MRSA. The present study indicates that the killing effect by the combinative treatment of morin and -lactam antibiotic is dependent on the PBP2a-mediated resistance mechanism.