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

Eucalyptus 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 E. urophylla clones and one E. urophylla × E. camaldulensis hybrid clone grown in Thailand. According to GC/MS analysis, the E. urophylla oils were mainly composed of 1,8-cineole, α -terpinyl acetate, β -caryophyllene, and spathulenol, while 1,8-cineole, α -terpinyl acetate, p -cymene, and γ -terpinene were mostly identified in the hybrid oil. All eucalypt oils exhibited a significant bacteriostatic effect against Gram-positive bacteria, Streptococcus pyogenes , Staphylococcus aureus , Listeria monocytogenes , and Bacillus cereus . Only the hybrid oil had an effect on all Gram-negative bacteria tested, including Salmonella typhi , Escherichia coli , Pseudomonas aeruginosa , and Enterobacter aerogenes . These oils have antibacterial properties that vary according to their terpenoid content. Only the hybrid oil had a potent antioxidant effect, with an IC 50 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 E. urophylla × E. camaldulensis hybrid essential oil are reported for the first time.