Biological and Phytochemical Profile of Volatile Oils from Romanian Mountain Flora

Background/Objectives: Essential oils (EOs) are complex mixtures extracted from plants. The aim of this study was to evaluate the chemical composition, antioxidant and antimicrobial properties, as well as in vivo toxicity against Drosophila melanogaster of the EOs extracted from Achillea millefolium L. (AM), Mentha longifolia L. (ML), and Thymus serpyllum L. (TS) from the mountains in Sibiu County, Romania. Methods: EOs were extracted by hydrodistillation. Their phytocompounds were determined by Gas Chromatography–Mass Spectrometry. Antioxidant potential was evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azinobis [3-ethylbenzothiazoline-6-sulfonic acid]-diammonium salt (ABTS), and ferric-reducing antioxidant power (FRAP) assays. Antibacterial action was measured against eight pathogenic bacteria. An in vivo D. melanogaster diet-dependent viability assay was conducted to assess the toxicity of the EOs. Results: Chamazulene (49.72%), pulegone (36.20%), and thymol (42.11%) were the most abundant constituents of the AM, ML, and TS EOs, respectively. All tested EOs exerted antioxidant activity, with TS EO being the most efficient in the DPPH (98.19% inhibition) and ABTS (1.82 mmol Trolox equivalent (TE)/mL EO) scavenging assays, while AM EO performed best in the FRAP assay (66.70 μmol TE/mL EO). Against TS EO, Bacillus subtilis was the most sensitive, with an inhibition zone of 37 mm. A dose-dependent decrease in the viability of D. melanogaster L. was observed when their diet was supplemented with AM, ML, and TS EOs. Conclusions: The analyzed EOs demonstrated antioxidant and antimicrobial effects, suggesting their potential applications in various fields. However, due to their toxicity, an in-depth analysis is required before introducing such products in therapy.