Isolation and characterisation of a novel xanthone with board-spectrum antibacterial activity from the roots of Allanblackia floribunda oliver (guttiferae)

Abstract

The increasing antimicrobial resistance of pathogens to existing therapeutic agents is currently a global health challenge, which led
us to a bioassay-guided investigation of Allanblackia floribunda Oliver (Guttiferae) roots for new antimicrobial constituents. The roots were extracted successively in Soxhlet with hexane, ethyl acetate and methanol. Isolation from the ethyl acetate extract was done using an open Column Chromatography, and Preparative Thin Layer Chromatography for purification. Structural elucidation was done using extensive 1-D and 2-D NMR, IR, MS data and physico-chemical properties. Antimicrobial activity and Minimum Inhibitory Concentration (MIC) were determined using agar-well diffusion and broth dilution methods, respectively. The root extracts, pooled fractions and isolated compound were tested on bacteria including Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Bacillus subtilis ATCC 6633 and clinical isolates of Salmonella typhi. The root extracts and pooled fractions (Al , Al A, F , A and A ) were active on the test bacteria with zones of 1 2 19 18 100 inhibition ranging from 12 to 20 mm. Fraction Al A showed highest consistent activity comparable to gentamycin standard and 2 yielded compound Al , a novel xanthone: 1, 2, 3, 4, 5, 8-hexahydroxy-9H-xanthen-9-one. The MICs in mg/mL of Al on the test 2 2 organisms are: S. aureus (0.75), E. coli (1.25), P. aeruginosa (1.13), B. subtilis (0.15) and S. typhi (0.08). These results justify the ethnomedicinal use of the root bark of Allanblackia floribunda in the treatment of diseases and the isolated xanthone has a high potential to become a chemotherapeutic agent for bacterial infections.