| Bacillus thuringiensis has been the most widely used microbial pesticides which is toxic to pests but safe for non-target organisms. This thesis launched research which aimed at problems that Bt pecticide may be effected by environmental microorganism from soil, the surface of roots and leaves of plants and the gut of insects. The contents of research included:Bt respectively interacted with three kinds of environmental microorganism including beneficial bacteria Bacillus subtilis, plant pathogenic fungi and cultivable gut bacteria of scarabs. The main results were as followed:Study on interaction of Bt with Bacillus subtilis:The results of cross marking method of tablet confrontation showed there existed three kinds of phenomenon on the interaction of92Bt strains preserved in our lab with B916-gfp:74percent of Bt strains were inhibited by B916-gfp,20percent of Bt strains could grow with B916-gfp together,6percent of Bt strains inhibited B916-gfp each other. The results of cup plate method of tablet confrontation showed that80percent of Bt strains were inhibited by B916-gfp. The results of co-culture in liquid medium method showed in the early stage of co-culture, Bt grew faster than B916-gfp. But in the late stage, we found that more than90percent on LB agar tablets were B916-gfp by spreading and culturing the mixture. We speculated that B916-gfp produced a large number of antibacterial substances in the late stage which caused Bt could not continue growing. The test of fluorescence intensity showed that Bt did not affect the expression of B916-gfp strain green fluorescent protein. Three methods of confrontation culture described above showed that the majority of Bt strains could be inhibited by Bacillus subtilis. This study not only made the interactions between Bt and B916-gfp clear, but also provided theoretical guidance that Bt should be avoided using with Bacillus subtilis together in the practical application.Study on interaction of Bt with plant pathogenic fungi:We gained a strain PS7-f4which had strong inhibitory effects on pathogenic fungi including Magnaporthe oryzae and Colletotrichum graminicola. The morphology of the strain, the identification of species, plasmid map and insecticidal activity analyses had been completed. The results showed PS7-f4strain (concentration:108cfu/mL)could cause73.3percent of Chilo suppressalis and80percent of Ostrinia furnacalis to death, and the survival performed growth inhibition. This study showed that some Bt strains had inhibitory effect on pathogenic fungi. Therefore, further screening of Bt strains resistance to pest and pathogenic fungi would reach the biocontrol effects of pesticide and plant disease-resistance. Study on interaction of Bt with cultivable gut bacteria of scarabs:we isolated32cultivable gut bacteria from Holotrichia oblita, Holotrichia parallela and Anomala corpulenta, and they were identified and divided into14genera. A confrontation culture analysis of the32isolates against three scarab specific Bt strains showed that the majority of the scarab gut bacteria had antibacterial activity against Bt strains. We speculated that this might be one reason that Bt had slow insecticide to scarabs. The Cry toxin stability analysis results showed that while several strains produced proteases capable of processing the scarab-specific toxin Cry8Ea, none were able to completely degrade it. The analysis of the sensitivity of scarab gut bacteria to antibiotics commonly used in laboratory results showed that the majority of scarab gut bacteria were sensitive to antibiotics. These results suggest that gut bacteria can potentially affect the susceptibility of scarabs to B. thuringiensis which should be considered when considering future control measures.To sum up, this thesis studied on interaction of Bt with three kinds of environmental microorganism and analyzed the influence caused by the biotic stress in the ecological environment which provided theoretical guidance of the improvement and application of Bt pesticides. |
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