Antimicrobial Activity of Aloe barbadensis Miller Leaf Gel Components

摘要

Methanolic extracts of Aloe barbadensis Miller inner leaf gel were fractionated by RP-HPLC and the resultant fractions were tested for inhibitory activity against a panel of bacteria and fungi. Five fractions were identified as having antimicrobial activity. Fraction 1 had the broadest antibacterial activity, being capable of inhibiting growth of both Gram-positive and Gram-negative bacteria as well as inhibiting growth of a nystatin resistant strain of the fungus Aspergillus niger. Fraction 1 had similar UV spectral properties as aloe emodin and was chromatographically identical to the pure compound. The other fractions tested were much more selective in their antimicrobial activities, being only capable of inhibiting the growth of specific Gram-negative rod bacteria. Two of these antimicrobial fractions were identified by ESI mass spectroscopy as being isomers of 8-C-β-D-[2-0-(E)-coumaroyl] glucopyranosyl -2-[2-hydroxy]-propyl-7-methoxy-5-methylchromone. Financial support of this work was provided by the School of Biomolecular and Physical Sciences, Griffith University. Introduction Bacterial resistance to antibiotics is increasingly becoming a concern to public health. Currently used antibiotic agents are failing to bring an end to many bacterial infections due to super resistant strains. For this reason the search is ongoing for new antimicrobial agents, either by the design and synthesis of new agents or through the search of natural sources for as yet undiscovered antimicrobial agents. Herbal medications in particular have seen a revival of interest due to a perception that there is a lower incidence of adverse reactions to plant preparations compared to synthetic pharmaceuticals. Coupled with the reduced costs of plant preparations, this makes the search for natural therapeutics an attractive option. Aloe barbadensis Miller (Aloe vera) has a long history of use as a therapeutic agent with many reported medicinal properties. Amongst its therapeutic properties, it has been shown to have anti-inflammatory activity (Azfal et al., 1991; Malterud et al., 1993) immunostimulatory activity (Ramamoorthy and Tizard, 1998), and cell growth stimulatory activity (Tizard et al., 1994; Rodriguez-Bigas, 1988). Furthermore, activity against a variety of infectious agents has been attributed to Aloe vera; for instance, antibacterial (Ferro et al., 2003), antiviral (Kahlon et al., 1991) and anti fungal (Kawai et al., 1998).Despite the therapeutic possibilities of this plant, there have been limited reports on the antimicrobial effects of isolated Aloe vera components. Ferro et al. (2003) have shown that Aloe vera leaf gel can inhibit the growth of the two Gram-positive bacteria Shigella flexneri and Streptococcus progenes. Specific plant compounds such as anthraquinones (Garcia-Sosa et al., 2006; Dabai et al., 2007) and dihhydroxyanthraquinones (Wu et al., 2006), as well as saponins (Reynolds and Dweck, 1999), have been proposed to have direct antimicrobial activity. Acemannan, a polysaccharide component from whole plant material, has been proposed to have indirect antimicrobial activity through its ability to stimulate phagocytic leukocytes (Pugh et al., 2001). Wang et al. (1998) have reported on the effect of the anthraquinone aloe emodin on arylamine N-acetyl transferase activity in Heliobacter pylori, and hence its antimicrobial activity. The purpose of this study is to test HPLC separated Aloe vera gel components against a comprehensive panel of microbes to characterise their antimicrobial activities. Furthermore, this report details the extraction, fractionation and partial identification of the antimicrobial leaf gel fractions. Materials and Methods Chemical ReagentsAloe emodin (Sigma, purity >95%) was prepared freshly before use by dissolving in distilled water to give a concentration of 500 μg ml -1 . Aloe vera juice was obtained from Aloe Wellness Pty Ltd, Australia and was stored at 4 o C until use. Aloe vera juice was used undi