This study was designed to assess the possibility of using medicinal plant extracts as β-lactamase inhibitors to control antibiotic-resistant Staphylococcus aureus. The susceptibilities of S. aureus ATCC 15564 (SAWT), ciprofloxacininduced S. aureus ATCC 15564 (SACIP), oxacillin-induced S. aureus ATCC 15564 (SAOXA), and clinically-isolated S. aureus CCARM 3008 (SACLI) to ampicillin were determined in the absence and presence of medicinal plant extracts, including Cleyera japonica (CJ), Carpinus laxiflora (CL), Euphorbia helioscopia (EH), Euscaphis japonica (EJ), Oenothera erythrosepala (OE), and Rosa multiflora (RM). The phenotypic change in the clear inhibition zones around ampicillin disc was observed for SAWT, SACIP, and SAOXA, indicating the production of ampicillinase. Compared to the controls, the MICs of ampicillin against SAWT, SACIP, and SAOXA were decreased from 4 to 0.5 ㎍/mL in the presence of CL, 16 to 4 ㎍/mL in the presence of RM, and 32 to 2 ㎍/mL in the presence of CL, EH, and RM, respectively. The medicinal plant extracts, OE, EJ, and CL, effectively inhibited the β-lactamase activities of SAWT (78%), SACIP (57%), and SAOXA (76%) when compared to the control. This results suggest that the medicinal plant extracts can be used as BLIs to control the antibiotic-resistant S. aureus.

Abstract
Introduction
Materials and Methods
    Bacterial strains and culture conditions
    Plant materials
    Hodge test
    Antimicrobial susceptibility assay
    β-Lactamase activity assay
    Statistical analysis
Results
    Antibiotic susceptibilities of SAWT, SACIP, SAOXA, andSACLI in the presence of medicinal plant extracts
    Inhibitory effect of medicinal plant extracts on the productionof β-lactamases by SAWT, SACIP, SAOXA, and SACLI
Discussion
    Abbreviations
References

• Juhee Ahn(Department of Biomedical Science and Institute of Bioscience and Biotechnology, Kangwon National University) Corresponding author
• Jirapat Dawan(Department of Biomedical Science, Kangwon National University)