Terpene constituents of the aerial parts, phenolic content, antibacterial potential, free radical scavenging and antioxidant activity of Callistemon citrinus (Curtis) Skeels (Myrtaceae) from Eastern Cape Province of South Africa

Background: Volatile oil from aromatic plants has been used by ancient Egyptians in embalming for the inhibition of bacterial growth and prevention of decay, Callistemon citrinus is used in traditional therapies for the treatment of bronchitis, cough, inflammation and as an antimicrobial herbs. This study examines the essential constituents of the volatile oils obtained from the aerial parts of the plant as well as its antioxidant activity, free radical scavenging, phenolic content and the antibacterial potential of the oils.

Methods: A portion of 500 g, 250 g and 150 g of the leaves, flowers and stems of this plant respectively were subjected to hydro-distillation process for three hours. The oils collected from the various plant parts were immediately subjected to GC-MS analysis. The overall phenolic content of the leaves oil, radical scavenging, antibacterial action and antioxidant activities of the essential oils of both the leaves and flowers of Callistemon citrinus were determined using standard methods, with free radical DPPH and ABTS as a reference antioxidant.

Results: Analyses of the three oils revealed a total of twenty-six components for the leaves oil representing 96.84% of the total oil composition, forty-one components for the flowers oil accounting for 98.92% of the whole composition and ten components for the stem oil amounting to 99.98% of the entire oil constituents. The dominant compounds in the leaves oil were eucalyptol (48.98%) and α-terpineol (8.01%), while α-eudesmol (12.93%), caryophyllene (11.89%), (-)-bornyl-acetate (10.02%) and eucalyptol (8.11%) were the main constituents of the flowers oil. In the same vein, the leading constituents in the stems oil were eucalyptol (56.00%) and α-pinene (31.03%). The antioxidant capacities of both the leaves and flowers oils of the plant were evaluated and their IC₅₀ were (1.49 and 1.13) for DPPH and (0.14 and 0.03) for ABTS assay respectively. The antibacterial activities of the oils from the (leaves and flowers) were also examined and were found to have wide range of activities against the bacterial strains used in this study.

Conclusion: Observations drawn from this experiment shows clearly that the leaves and flowers of Callistemon citrinus possess phenolic compounds and cyclic ether of several pharmacological behaviors.