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Shikimic acid (SA) is an important kind of organ acid.With the sudden outbreak of flu, SA has aroused unprecedented attention by scholars and socials, because it is a key material to synthesis of antiviral drug Tamiflu@, which can treat antiviral infections such as bird flu and swine flu.In the biological studies, SA has been reported to have anti-inflammatory, analgesia, antioxidant, antithrombotic and antibacterial activities.Aim of the study SA has been reported to possess antibacterial activity against Staphylococcus aureus, whereas the mode of action of SA is still elusive.In order to fully understand the mode of action and better application of SA, we investigated the effects of SA acid on cell membrane of S.auresus to shed light on the mechanism in its antibacterial activity.Materials and methods SA was obtained from pine needles of C.deodara.The minimum inhibitory concentration (MIC) values of SA were estimated by agar dilution method.The antibacterial mechanism of SA was evaluated the cell membrane permeability, membrane potential, membrane integrity, cell morphology, membrane fluidity and membrane protein.The assay of the potassium efflux was measured using atomic absorption spectrometer.The nucleotide leakage from S.aureus was measured by the microplate reader.The changes of membrane potential were detected the fluorescence intensity of bis-(l,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)).The effect of SA on S.aureus membrane integrity was determined using the LIVE/DEAD BacLight bacterial viability kit.The changes of cell morphology were observed by transmission electron microscopy (TEM).Membrane fluidity was determined by measuring fluorescence polarization of 1,6-diphenyl-l,3,5-hexatriene (DPH).The effect of SA on membrane protein was detected the emission spectra by using the fluorescence spectrophotometer.Results The MIC of SA was detected to be 2.5 mg/mL against S.aureus ATCC 6538.SA treated S.aureus showed the leakage of massive K+ and nucleotides and the change of membrane potential.These observations suggest that SA possibly disrupt the cell membrane permeability to show the antibacterial activity.Transmission electron microscopic observations of the morphologies of S.aureus treated with SA further confirm that SA can disrupt the cell membrane and membrane integrity.Meanwhile, SA was found to be capable of reducing the membrane fluidity of the S.aureus cell, which indicated that SA may damage the structure of cell membrane lipid bilayers and hinder the cell modulation of membrane composition.Moreover, the fluorescence experiments indicated that SA could quench fluorescence of Phe residues of the membrane proteins, thus demonstrating that SA can bind to membrane proteins of S.aureus.Therefore, the results showed the antibacterial activity of SA against S.aureus could be caused by the interactions of SA with membrane proteins and lipids of the bacteria, resulting in causing cell membrane dysfunction and bacterial damage or even death.Conclusions The experiment data showed that the antibacterial activity of SA against S.aureus may be attributed to its interaction with the cell membrane, which consequently increases membrane permeability and destroys membrane integrity, eventually inducing growth inhibition and bacterial death.This study reveals the potential use of SA as an antibacterial agent.