The Role of Endophytic/Epiphytic Bacterial Constituents in the Immunostimulatory Activity of the Botanical, Astragalus membranaceus

Astragalus membranaceus is a staple of Traditional Chinese Medicine being one of the oldest medicinal herbs listed in the material medica of Chinese herbal medicine. Chinese herbalists have used Astragalus to help the human body fight a variety of diseases. Modern herbalists utilize Astragalus primarily as an immunostimulant to prevent common infection and aid in the recovery following infection. Historically, the biological activities associated with Astragalus have been accounted for, at least in part, to several constituents present in the botanical including saponins and polysaccharides. We propose that in addition to these constituents, compounds from endophytic (or epiphytic) bacteria present in (or on) the roots of Astragalus may have an important biological role. Lipopolysaccharides and lipoproteins are major components of Gram-negative bacteria and highly potent activators of the innate immune response. Our data supports a direct correlation between the level of immune gene induction and the level of lipopolysaccharides/lipoproteins present in the Astragalus extract. We demonstrate that extracts from Astragalus specifically activate Toll-like and NOD-like receptors involved in the recognition and response to bacterial constituents and that removal of the lipopolysaccharide/lipoprotein from the Astragalus extract reduced the level of this response. The results support that many immune enhancing botanicals have established a symbiotic relationship with Gram-negative bacteria and that the immune enhancing effect of these botanical extracts on the body may not only be due to endogenous plant compounds, but endophytic (or epiphytic) bacterial components as well.

Keywords: Astragalus membranceus; Rhizobium; botanical; endophytic bacteria; immune; lipopolysaccharide; lipoprotein.