Efficient Enrichment of Total Flavonoids and Antibacterial Activity of the Ethyl Acetate Fraction of Croton blanchetianus Baill. (Euphorbiaceae) Leaves

Background/Objectives: This study investigated the flavonoid enrichment and antimicrobial activity of the ethyl acetate fraction (EAF) obtained from Croton blanchetianus (Euphorbiaceae) leaves against Staphylococcus aureus, including the methicillin-resistant strains (MRSA) that were isolated, as well as its possible mechanism of action. Methods: Croton blanchetianus leaves were extracted with ethanol:water (50%), then the extract was spray-dried and partitioned (8×) with ethyl acetate. Phytochemical analysis was performed using thin layer chromatography (TLC), while antibacterial activity was conducted using minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) methods. Results: Chemical profiling (TLC) confirmed multiple flavonoid bands and the presence of hyperoside; the total flavonoid content in the EAF reached 25.3% (≈2.28× the spray-dried extract and 6.65× the aqueous fraction). The MIC and MBC assays against S. aureus ATCC 29213 and six clinical isolates showed an MIC of 4–32 μg/mL and an MBC of 16–64 μg/mL for EAF. The combination of EAF with chloramphenicol showed a complete synergistic effect for S. aureus ATCC 29213 and S. aureus UFPEDA 705, a partial effect for S. aureus UFPEDA-659 and S. aureus UFPEDA-671, antagonistic effect for S. aureus UFPEDA 731 and S. aureus UFPEDA 802, and no effect for S. aureus UFPEDA-691. Growth curves indicated time- and concentration-dependent inhibition. Membrane integrity assays revealed K+ efflux and release of DNA/RNA and proteins, suggesting bacterial membrane destabilization as a likely mechanism. Conclusions: The flavonoid-rich fraction of C. blanchetianus exhibits potent anti-S. aureus activity, including MRSA. Furthermore, it was observed that EAF has a synergistic effect with chloramphenicol and acts through membrane damage, making it a candidate for a phytoderived adjuvant in antimicrobial therapies.