Abstract: 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, ?-terpinyl
acetate, ?-caryophyllene, and spathulenol, while 1,8-cineole, ?-terpinyl acetate, p-cymene, and ?-
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 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 E. urophylla ? E. camaldulensis hybrid
essential oil are reported for the first time.