Antimicrobial Evaluation of Anthraquinones and Preanthraquinone from the Root Extract of Aloe kefaensis

The aim of this study was to isolate antimicrobial compounds from the roots of Aloe kefaensis, a plant endemic to Ethiopia and used to treat various microbial infections by traditional healers. The air-dried and powdered plant material was sequentially extracted with petroleum ether, dichloromethane, acetone, and methanol. Then, each solvent extract was evaluated for its in vitro antimicrobial activity against four bacterial (Escherichia coli, Bacillus cereus, Salmonella typhimurium, and Staphylococcus aureus) and one fungal (Candida albicans) strains using the agar disk diffusion method. Superior antimicrobial activity was exhibited against all the strains by dichloromethane extract, with the highest activity observed against S. typhi (inhibition zone diameter of 23.0 mm at 200 mg/mL). Due to similarity in their TLC profile, the acetone and dichloromethane extracts were combined and subjected to silica gel column chromatography for fractionation and isolation of the compounds. Separation of these extracts using silica gel column chromatography resulted in four anthraquinones: deoxyerythrolaccin (1), chrysophanol (2), laccaic acid D-methyl ester (3), and 3, 8-dihydroxy-1-methylanthraquinone-2-carboxylic acid (4) and one preanthraquinone, aloesaponol II (5). The structure of these compounds was established using NMR (1D and 2D) spectroscopic analysis and comparison with reported literature data. The isolated compounds were evaluated for antimicrobial activity and showed varying degrees of potency. Compounds 2 and 4 showed the highest activity against Salmonella typhimurium at 10 mg/mL, with a zone of inhibition of 28.5 and 25.0 mm, respectively, in comparison to gentamicin (26.0 mm inhibition zone diameter at 10 mg/mL). Therefore, this strong antimicrobial activity of the extracts and isolates supports the traditional usage of Aloe kefaensis to treat microbial diseases.