| Usnea belongs to Ascolichens,Lecanorales,Usneaceae. Usnea longissima Ach. from Shejila Mountain in Tibet was studied in this paper. A new method determining the content of usnic acid was established; the best extracion procedure by ethanol was selected; the antimicrobial activities of usnic acid was assayed. At the same time, we have investigated the vertical distribution of U. longissima in Shejila Mountain in Tibet and the species of its hosts for the first time.The method of thin layer chromatography-ultraviolet spectrophotometry(TLC-UVS) was used to determine the content of usnic acid, which was extracted with EtOAC refluxing. We also employed singal factor experiments and orthogonal experiment to select the best condition of EtOH refluxing extraction for usnic acid. Then the antimicrobial activities of usnic acid against Escherichia coli, Staphylococcus aureus and several mould and yeast was assayed by filtering paper;meanwhile, the inhibitory ability of the decay of fresh meat, bean curd and milk was discussed.The main resuls are as follows: there is a good reproduction and a high precision (RSD=3.44%, the ratio of recovery is 96.1%) of determining the content of usnic acid in the same plate by TLC-UVS; the optimum condition of extraction is the ratio of 1(U. longissima) : 13(95% ethanol) and refluxing at 80℃for 2 times and 3 hours per time; usnic acid not only has obviously inhibitory effects on the growth of bacterium, mould and yeast, especially the strongest inhibitory effects against Escherichia coli and Staphylococcus aureus, but also has preservation effects on the decay of fresh meat, bean curd and milk; U. longissima in Shejila Mountain in Tibet mainly distributes between 3200m and 3800m(height above sea level).According to our preliminary investigation, the hosts on which U. longissima grows belong to 11 families 17 genera 18 species, mainly Abies georgei .So we can conclude that TLC-UVS is a precise, simple and easily operated method to determine the content of usnic acid. In addition, compared with benzoic acid in the same condition, the inhibitory and preservation effects of usnic acid are a little worse than these of benzoic acid, but usnic acid is a natural antimicrobial and preservative, which can be exploited in field of natural food preservatives in the future. |
