Elemental, phytochemical analysis and determination of antibacterial, antioxidant, and cytotoxic activities of leaves extracts of eleven Eucalyptus species

Abstract

The genus Eucalyptus has attracted considerable attention from natural product specialists exploring new phytochemicals for therapeutic purposes due to their possible biological activities. This was stimulated by current findings that Eucalyptus species interrupt the propagation of the recently tormenting COVID-19 virus. Eleven Eucalyptus leaf extracts were subjected to in vitro antimicrobial studies against four bacterial strains: Staphylococcus aureus, Streptococcus pyogenes, Staphylococcus lugdunensis and Staphylococcus epidermis using the broth serial dilution assay. The potential of the extracts to inhibit free radicals 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) was investigated, indicating antioxidant properties. Furthermore, the cytotoxicity test was performed against the human embryonic kidney (HEK293-T) cell line using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Phytochemicals were identified and characterized using Fourier transform infrared spectroscopy (FTIR) and one-dimensional gas chromatography mass spectrometry (GC–MS). Moreover, the microwave plasma atomic emission spectroscopy (MP-AES) coupled with inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to evaluate the multielemental composition of the extracts. The MP-AES and ICP-OES techniques in this work displayed accuracy and sensitivity for multielement determination by identifying P, B, Ca, Zn, Cu, Mg, Fe, Mn, Na, and K as major constituents in Eucalyptus extracts. The extracts generally had strong antioxidant and antibacterial properties, with E. viminalis (IC50 = 2.91 ± 0.22 µg/mL) demonstrating strong action against the ABTS radical. E. nitens and E. dunnii also showed highest activity against DPPH and ABTS, with IC50 values < 10 µg/mL. Additionally, with IC50 values greater than 20 µg/mL, the extracts demonstrated relative non-toxicity against HEK293-T cells. The existence of the bioactive chemicals found by FTIR and GC–MS may be the cause of the observed biological activity. Because Eucalyptus plant species have the highest antioxidant activity and the most inhibitory action against the majority of Streptococcus bacterial strains, they can be employed as a therapeutic treatment to treat infections caused by Streptococcus.