Liquid and Vapor Phase of Four Conifer-Derived Essential Oils: Comparison of Chemical Compositions and Antimicrobial and Antioxidant Properties

Abstract

In this study, the chemical composition of the vapor and liquid phase of <i>Pinus cembra</i> L., <i>Pinus mugo</i> Turra, <i>Picea abies</i> L., and <i>Abies Alba</i> M. needles essential oils (EOs) was investigated by Headspace-Gas Chromatography/Mass Spectrometry (HS-GC/MS). In the examined EOs, a total of twenty-eight components were identified, most of which belong to the monoterpenes family. α-Pinene (16.6-44.0%), β-pinene (7.5-44.7%), limonene (9.5-32.5%), and γ-terpinene (0.3-19.7%) were the most abundant components of the liquid phase. Such major compounds were also detected in the vapor phase of all EOs, and α-pinene reached higher relative percentages than in the liquid phase. Then, both the liquid and vapor phases were evaluated in terms of antibacterial activity against three Gram-negative bacteria (<i>Escherichia coli</i>, <i>Pseudomonas fluorescens</i>, and <i>Acinetobacter bohemicus</i>) and two Gram-positive bacteria (<i>Kocuria marina</i> and <i>Bacillus cereus</i>) using a microwell dilution assay, disc diffusion assay, and vapor phase test. The lowest Minimum Inhibitory Concentration (MIC) (13.28 mg/mL) and Minimal Bactericidal Concentration (MBC) (26.56 mg/mL) values, which correspond to the highest antibacterial activities, were reported for <i>P. abies</i> EO against <i>A. bohemicus</i> and for <i>A. alba</i> EO against <i>A. bohemicus</i> and <i>B. cereus</i>. The vapor phase of all the tested EOs was more active than liquid phase, showing the inhibition halos from 41.00 ± 10.15 mm to 80.00 ± 0.00 mm for three bacterial strains (<i>A. bohemicus</i>, <i>K. marina</i>, and <i>B. cereus</i>). Furthermore, antioxidant activities were also investigated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azinobis (3- ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) assays, and a concentration-dependent antioxidant capacity for all EOs was found. <i>P. mugo</i> EO showed the best antioxidant activity than the other Pinaceae EOs. The four Pinaceae EOs could be further investigated for their promising antibacterial and antioxidant properties, and, in particular, α-pinene seems to have interesting possibilities for use as a novel natural antibacterial agent.