This research aimed to study the potential synergistic effect of Hydnophytum formicarum tuber and Vatica diospyroides Symington cotyledon extracts combined with ampicillin on Staphylococcus aureus ATCC 25923 and Escherichia coli ATTC 25922. The antibacterial efficacy was assessed using agar well diffusion, minimum inhibitory concentration (MIC), fractional inhibitory concentration index (FICI), and a flow cytometry assay. The results indicate synergy in antibacterial activity of combined V. diospyroides cotyledon extract with ampicillin, which was more effective than H. formicarum tuber extract against both bacterial species (clear zone sizes 49.33 ± 0.58 and 30.67 ± 1.15 mm for S. aureus and E. coli, respectively). The MIC values for the combination against these bacteria were 1,000:3.125 and 500:3.125 µg/ml, respectively. The FICI values of the combination were 1.06 and 0.375, demonstrating independent and synergistic effects, respectively. The results of flow cytometry indicated that the combined extract-antibiotic inhibited bacteria by acting on cell membrane and by granularity disruptions, with the cell membrane representing the primary target followed by lost granularity. Vatica diospyroides cotyledon extract may increase the efficacy of ampicillin and reduce 8-fold the required antibiotic concentration. These results support a drug discovery program assessing V. diospyroides combined with antibiotics as therapy against resistant bacteria.
Key words: Checkerboard, Flow cytometry, Hydnophytum formicarum, Synergistic, Vatica diospyroides
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