Synergistic Antibacterial Activities of Bioactive Compounds from Streptomyces sp. RS2 in Combination with Vancomycin against Staphylococcus aureus

Two bioactive compounds, namely tetrangulol methyl ether (1) and 2-phenylacetamide (2) were produced by Streptomyces sp. RS2, and were also active in dealing with Methicillin-resistance S. aureus (MRSA) and Staphylococcus aureus. The objective of this research was to determine synergistic antibacterial combinations between the active compounds with an antibiotic against S. aureus. The combined effects analysis was performed by the method known as checkerboard in order to find the Fractional Inhibitory Concentration (FIC) indexes, and all synergistic combinations were confirmed using time kill kinetic analysis. Synergistic effects were observed when compound 1 and 2 were combined with vancomycin (VA) against S. aureus ATCC 25923. FIC indexes of compounds 1+VA, 2+VA and 1+2 combinations were 0.313, 0.500 and 0.375, respectively. The result of time-kill kinetic analysis showed that the compounds 1, 2 and VA, and compound combinations 1+VA, 2+VA and 1+2 exhibited synergistic activity against S. aureus ATCC 25923 within 4, 6 and 24 h. In addition, the effects of the active compounds on the tested S. aureus and MRSA were observed by Scanning Electron Microscopy (SEM). The results illustrate that bacterial cells were slightly changed and decreased after incubation with the compounds 1 and 2 and the 1+2 combination. These results demonstrate that the active compound exhibited synergistic interaction with S. aureus ATCC 25923, and the two active compounds will be studied particularly for the role or function of the two bioactive compounds in the future.