OBJECTIVE To evaluate the synergistic effect of rosmarinic acid(RA)with antibiotics against Staphylococcus aureus(S.aureus)and MRSA and to identify the possible mechanism of action responsible for synergism if any.METHODS The antibacterial activity of Rosmarinic acid was studied for its zone of inhibition by agar well diffusion and MIC determination by liquid broth dilution technique against MRSA and VRSA.The synergistic effect of RA with antibiotics like amoxicillin,ofloxacin and vancomycin was evaluated by Broth checker board method and further confirmed with time kill kinetic studies.Microbial Surface Components Recognizing Adhesive Matrix Molecules(MSCRAMMs)were isolated by protein precipitation technique and its expression was studied by SDS PAGE.Further RA was evaluated for its beta lactamase inhibition property.RESULTS Rosmarinic acid exhibited antibacterial activity against S.aureus and MRSAby showing a MIC value of 0.8mg·mL-1 against S.aureus and 10mg·mL-1 against MRSA.Rosmarinic acid at 1/4X MIC value reduced the MIC of vancomycin,amoxicillin and ofloxacin by 1/4 times against S.aureus.But against MRSA,vancomycin was found to be synergistic.All the synergistic combinations have shown FIC value of 0.5.In order to measure the kinetics of the anti-bacterial activity,the bacterial growth rate with RA,antibiotics and synergistic combinations against S.aureus and MRSA was studied.It is observed that the synergistic combinations showed better time kill kinetics as compared to RA and antibiotics.Further RA could able to destruct the cell surface proteins MSCRAMMs which was studied by SDS PAGE.RA was also found to showβlactamase inhibiting property.CONCLUSION It is concluded that RA possess antibacterial activity but at a very higher concentration(in mg/mL)against S.aureus and MRSA.However it shows synergism with antibiotics and could able to reduce the MIC of antibiotics.Thus RA could be developed as an adjuvant for antibiotics against S.aureus and MRSA caused infections.Further studies are needed to identify the mechanism for its synergism with antibiotics. OBJECTIVE To evaluate the synergistic effect of rosmarinic acid (RA) with antibiotics against Staphylococcus aureus (S. aureus) and MRSA and to identify the possible mechanism of action responsible for synergism if any. METHODS The antibacterial activity of Rosmarinic acid was studied for its zone of inhibition by agar well diffusion and MIC determination by liquid broth dilution technique against MRSA and VRSA.The synergistic effect of RA with antibiotics like amoxicillin, ofloxacin and vancomycin was evaluated by Broth checker board method and further confirmed with time kill kinetic studies. Microbial Surface Components Recognizing Adhesive Matrix Molecules (MSCRAMMs) were isolated by protein precipitation technique and its expression was studied by SDS PAGE. Fermentation RA was evaluated for its beta lactamase inhibition property .RESULTS Rosmarinic acid exhibited antibacterial activity against S. aureus and MRSA by showing a MIC value of 0.8 mg · mL-1 against S. aureus and 10 mg · mL-1 of MRSA.Rosma rinic acid at 1 / 4X MIC value reduced the MIC of vancomycin, amoxicillin and ofloxacin by 1/4 times against S. aureus. But against MRSA, vancomycin was found to be synergistic. All the synergistic combinations have shown FIC value of 0.5. order to measure the kinetics of the anti-bacterial activity, the bacterial growth rate with RA, antibiotics and synergistic combinations against S.aureus and MRSA was studied. It is observed that the synergistic combinationsÎollection showed time kill kinetics as compared to RA and antibiotics .Further RA could able to destruct the cell surface proteins MSCRAMMs which was studied by SDS PAGE. RA was also found to showβlactamase inhibiting property. CONCLUSION It isscribing that RA possess antibacterial activity but at a very higher concentration (in mg / mL) against S.aureus and MRSA.However it shows synergism with antibiotics and could able to reduce the MIC of antibiotics.Thus RA could be developed as an adjuvant for antibiotics against S.aureus and MRSA caused infections.Further studies are needed to identify the mechanism for its synergism with antibiotics.