Antibacterial mechanism of aqueous extract of Lithocarpus pachylepis leaf against methicillin-resistant Staphylococcus aureus

【Objective】 To make clear antibacterial effect and mechanism of aqueous extract of Lithocarpus pachylepis leaf（AELL）against methicillin-resistant Staphylococcus aureus（MRSA），so as to provide theoretical basis for developing safe，efficient，environmentally friendly，low-toxic and residue-free anti-MRSA drugs.【Method】 Minimal inhibitory concentration（MIC）and minimal bactericidal concentration（MBC）of AELL against MRSA were measured by broth microdilution method and plate colony counting method.Growth curve of MRSA after AELL treatment was drawn by turbidimetry.Biofilm clearance，fibrinogen adhesion rate，respiratory metabolism activity，soluble protein content，DNA content，cell cycle distribution，apoptosis rate，intracellular reactive oxygen species（ROS）level and morphological and structural characteristics of MRSA after AELL treatment were measured by microplate reader，flow cytometry and scanning electron microscope.【Result】AELL could inhibit or kill MRSA by affecting its logarithmic growth phase，and its MIC and MBC were both 0.15625 mg/mL.Compared with the blank control group，the biofilm content of MRSA after AELL treatment decreased significantly.With the increase of AELL concentration，relative adhesion rate of MRSA decreased gradually from 47.486% to 5.440%.With the prolongation of AELL treatment time，respiratory metabolic activity of MRSA decreased gradually，and respiratory chain dehydrogenase activity was significantly lower than that of the blank control group，which indicated that AELL might inhibit respiratory chain dehydrogenase activity of MRSA to kill or break the strain.After AELL treatment，soluble protein of MRSA decreased obviously，and distribution of MRSA cell cycle was disturbed，making MRSA stay in S phase and inhibiting DNA synthesis；apoptosis rate and intracellular ROS level of MRSA increased gradually with the increase of AELL concentration；morphological structure of MRSA were irregular，atrophied and deformed，the cells collapsed，the intercellular cracks appeared，and the cell walls were severely bent and deformed.【Conclusion】 AELL is potential to be developed as an anti-MRSA veterinary drug and botanical fungicide.Its bacteriostasis mechanism is to remove MRSA biofilm，reduce its adhesion to fibrinogen，inhibit protein and DNA production，hinder respiratory metabolism，delay cell reproduction cycle and promote lipid peroxidation in bacteria，leading to apoptosis of bacteria.