Structural elucidation of some antimicrobial constituents from the leaf latex of Aloe trigonantha L.C. Leach

Abstract

Background: The incidents of drug resistant microorganisms and the need of treatments for newly emerging pathogens are of great concern to the global community. Our ability to treat infectious diseases is dependent on the development of new pharmaceuticals, and one potential source being medicinal plants with traditional claims. The leaves of Aloe trigonantha L.C. Leach, an endemic Ethiopian plant, are locally used for the treatment of infectious and inflammatory diseases. This study explores the potential of the latex of this plant and compounds isolated thereof for their in vitro antibacterial and antifungal properties.

Methods: Analytical RP-HPLC and silica gel preparative TLC were used for identification and isolation of active constituents, respectively. Characterization of the compounds was based on UV, IR, HR-ESIMS, (1)H and (13)C NMR, and 2D-NMR spectral assignments. Antimicrobial activity studies were carried out against 21 pathogenic bacterial and 4 fungal strains using the disk diffusion method. Minimum inhibitory concentrations (MICs) were determined by the broth dilution method.

Results: A C-glycosylated chromone identified as aloesin, and three C-glycosylated anthrones characterized as 8-O-methy-7-hydroxyaloin A/B, aloin A/B and aloin-6'-O-acetate A/B were isolated. The latex and isolated compounds exhibited in vitro antibacterial activity against the tested pathogens. In some cases the activity of the isolated compounds (MIC = 10 μg/mL) was comparable with that of the standard drug ciprofloxacin, particularly against some of the Gram-negative bacterial strains tested. However, their activity towards the fungal pathogens tested was relatively weaker showing maximum activity against Candida albicans with MIC value of 400 μg/mL.

Conclusion: The present findings can be used for further research aimed at the development of new antibacterial agents, and may also justify the ethnomedicinal claim of the plant for the treatment of infectious diseases.

Fig. 1

HPLC chromatogram of the leaf latex of Aloe trigonantha. Column: RP-18 (250 mm x 3.2 mm, 5 μm diameter particles); flow rate 1 mL/min; UV detection: 254 nm; Solvent system: 30 % to 60 % linear gradient of methanol in water over 25 min, 3 min isocratic, 100 % in 2 min, 4 min isocratic

Fig. 2

Structures of (a) aloesin (1); (b) 8-O-methyl-7-hydroxyaloin A/B (2), aloin A/B (3) and aloin-6’-O-acetate A/B (4)