Bioactive Potential of <i>Origanum vulgare</i> Rhizomes: Phenolic Composition, Antioxidant, Antibacterial, and Cytotoxicity Profiles

Utilization of plant by-products contributes to the efficient use of natural resources and expands the range of bioactive materials of botanical origin. Oregano (<i>Origanum vulgare</i> L.) is widely recognized for its active metabolites; however, its underground parts have received little attention. In this study, 14 phenolic compounds were identified in water extracts of oregano leaves (OVL) and rhizomes (OVR) using LC-MS/MS-DAD. Both extracts contained rosmarinic acid, oreganol C, and caffeic acid, while OVL also included oreganol A, oreganol B, and luteolin-7-<i>O</i>-diglucuronide. Remarkably, OVR was revealed as a previously unreported natural source of salvianolic acid A. The antioxidant potential, evaluated by DPPH, ABTS, and H₂O₂ scavenging assays, demonstrated comparable activities of OVL and OVR. Antimicrobial testing (broth microdilution and isothermal microcalorimetry) confirmed inhibitory effects against <i>Staphylococcus aureus</i> (MSSA, MRSA), <i>Proteus mirabilis</i>, and <i>Enterococcus faecalis</i>. Notably, OVR showed stronger inhibition of staphylococcal metabolic activity (≥ 0.75 mg/mL) than OVL (≥ 5.0 mg/mL). Cytotoxicity assessed by MTT assay on HaCaT keratinocytes indicated no significant reduction in cell viability at concentrations ≤ 100 μg/mL, supporting the biocompatibility of both extracts. The rhizome extract further showed a safety profile comparable to that of the leaf extract, supporting its potential for further biomedical use. The present study demonstrates that oregano rhizomes represent an unexplored botanical source of antioxidant and anti-staphylococcal compounds, expanding current knowledge on the phytochemical potential of this species.