Effect of Different Arnica montana L. Plant Parts on the Essential Oil Composition, Antimicrobial Activity, and Synergistic Interactions with Antibiotics

<i>Arnica montana</i> L. (mountain arnica) is a medicinal plant with diverse biological activities commonly used in pharmacy and cosmetics. The attributes of <i>A. montana</i> are related to e.g., the concentration and chemical composition of its essential oils (EOs). Therefore, the objective of this study was to: (i) characterize the chemical composition of EOs obtained from mountain arnica flower heads, rhizomes, and roots used as a pharmacopoeial material, (ii) demonstrate the effects of particular EO types and their combinations on antibacterial activity, and (iii) demonstrate the effect of the presence of <i>A. montana</i> EOs and their combinations with commercial antibiotics on their antibacterial activity. Essential oils obtained by hydrodistillation from different parts of <i>A. montana</i> were screened for their chemical composition and antibacterial properties. The chemical composition of the EOs was determined using the GC-MS technique. E-caryophyllene, caryophyllene oxide, germacrene D, farnesyl acetate, and dodecanal were the main components of the EO distilled from the flower heads. In turn, 2,5-dimethoxy-p-cymene, 2,6-diisopropylanisole, p-methoxyheptanophenone, and thymol methyl ether were the main molecules detected in the EO from the <i>A. montana</i> rhizomes and roots. The data clearly indicate that the presence of mountain arnica EO alone and in the interaction with commercial antibiotics (amoxicillin, ciprofloxacin, metronidazole) has a beneficial effect on their antibacterial activity.