Selection And Breeding Of High-yielding Avermectins Strains

The avermetins are a group of closely related macrocylic lactones with exceedingly high activity against helminths and anthropods. This paper review the biosyntheticpathway of the avermectins and the organization of the biosynthetic gene cluster which many groups have analysed and cloned. According to the cloned gene cluster, people can use genetical technology to obtain the genetical strains which can produce more potent and non-toxical avermectins and its derivatives. An extensive program of chemical modificaton can also be carried out to discover more active avermectins. Avermectin has been into industrial produce as one of wide-scope pesticide.Four types of colony ,powdery gray, gray with white blot ,white and bald were isolated from Streptomyces avermitilis among them only the powdery gray one produced the highest avermectin. The Gray with white blot one only produced 70%-80% less than the powdery gray. And the others produced none.The effects of various mutagenic agents on the productivity of avermectin by Slreplomyces avermilitis were evaluated in this work. Three different mutagenic breeding methods were developed, such as NTG, UV, Ion implatation. Ion implantation were most efficient for positive mutation of S. avermilitis. The distribution fractions of mutants with higher avermectin titer than the original had correlation with implantation dose. After ion implantation mutation there were many types of morphology of colony , we stdudied the relationship between the colony shape and the ability of yielding , found that gray and protuberant colony had the highest titer.After two rounds of cyclic screening, we obtained eight mutant strains which produced 16.9-63.2% more avermectin than that of original strain. And they were all genetic stability. Its avermectin titer attained 3900ug/ml in shaking flask. B1a ratio of Strain N10 attained 29.9%.Several factors affecting avermectin production were studied. The highest avermectin yielding were obtained when the fermentation conditions were as follows: temperature 28@, inoculation 12%, rotation 210 r.p.m, seed age of 30h. It was found that the ammonia ion could distinctly inhibit biosynthesis of avermectin and the mechanism of this phenomenon was investigated. K2HPO4 and MgSO4 did not inhibit orincrease the avermectin yielding.Comprehensive studies were carryied out in composing the fermentation medium. An optimizing medium was obtained via orthogonal experiment. The optimized concentration : cornsteep 2g/l, corn starch 90g/l and soybean cakepowder 10g/l.