Variation in the Antibacterial and Antioxidant Activities of Essential Oils of Five New Eucalyptus urophylla S.T. Blake Clones in Thailand

Abstract

<i>Eucalyptus</i> oils are widely used for a variety of purposes. This study investigates the terpenoid compositions and antibacterial and antioxidant activities of eucalypt leaf oils extracted from four <i>E. urophylla</i> clones and one <i>E. urophylla</i> × <i>E. camaldulensis</i> hybrid clone grown in Thailand. According to GC/MS analysis, the <i>E. urophylla</i> oils were mainly composed of 1,8-cineole, <i>α</i>-terpinyl acetate, <i>β</i>-caryophyllene, and spathulenol, while 1,8-cineole, <i>α</i>-terpinyl acetate, <i>p</i>-cymene, and <i>γ</i>-terpinene were mostly identified in the hybrid oil. All eucalypt oils exhibited a significant bacteriostatic effect against Gram-positive bacteria, <i>Streptococcus pyogenes</i>, <i>Staphylococcus aureus</i>, <i>Listeria monocytogenes</i>, and <i>Bacillus cereus</i>. Only the hybrid oil had an effect on all Gram-negative bacteria tested, including <i>Salmonella typhi</i>, <i>Escherichia coli</i>, <i>Pseudomonas aeruginosa</i>, and <i>Enterobacter aerogenes</i>. These oils have antibacterial properties that vary according to their terpenoid content. Only the hybrid oil had a potent antioxidant effect, with an IC₅₀ value of 4.21 ± 0.35 mg/mL for free radical (DPPH) scavenging. This oil's antioxidant effect may be a result of the phenolic terpenoids, thymol and carvacrol. As a result, these oils may be a novel source of antibacterial and antioxidant agents. Additionally, the antibacterial and antioxidant capabilities of the <i>E. urophylla</i> × <i>E. camaldulensis</i> hybrid essential oil are reported for the first time.