LC-MS/MS profiling, total phenolic and flavonoid contents, and antibacterial activity of Allium cepa L. peel extract against Staphylococcus epidermidis

Context: Allium cepa L. (shallot) peel contains secondary metabolites, including alkaloids and flavonoids, with potential antibacterial activity against Staphylococcus epidermidis. Aims: To examine the antibacterial activity and minimum inhibitory concentration (MIC) of A. cepa peel extract against S. epidermidis, quantify its total phenolics and flavonoids content, and identify its active compounds using LC-MS/MS. Methods: Extraction was conducted using ethyl acetate and n-hexane. Antibacterial activity was assessed by the disc diffusion method with clindamycin as a positive control. MIC was determined by macrodilution and analysed using a UV-Vis spectrophotometer. Total phenolics and flavonoids were measured using colorimetric assays, and LC-MS/MS performed compound profiling. Results: The ethyl acetate extract inhibited S. epidermidis with zones of 10.27–16.04 mm (n = 4); concentrations of 200,000–500,000 μg/mL were intermediate, and lower doses were resistant. Although inhibition was lower than that of the positive control (clindamycin, 21.64 mm), the MIC was 2,500 μg/mL (n = 3). The n-hexane extract remained inactive across all concentrations. The extract contained 30.75 ± 1.56 mgGAE/G of phenolics and 22.44 ± 1.05 mgQE/G of flavonoids. LC-MS/MS identified rhamnetin (RT 16.26 min), nobiletin (RT 16.67 min), and casticin (RT 15.77) as dominant flavonoids likely responsible for the antibacterial activity. Conclusions: Rhamnetin, nobiletin, and castisin were identified as candidate antibacterial compounds in the ethyl acetate extract of A. cepa peel.