Preliminary Study On Antimicrobial Activity Of Bacillus From Termite Nest And Its Mechanism

Long term overuse of chemical pesticides is easy to cause disease resistance,environmental pollution,damage of beneficial microbes in the soil,and pesticide residues.With its advantages of green environmental protection and economic sustainability,microbial biopesticides can meet the needs of agricultural production for reducing the adverse impact on the environment,and it gets more and more attention.Social insects are large groups in nature.They build their own nests to provide habitat and breeding grounds.Insects live in the nests for a long time.The nests not only offer a place to live,but also provide a suitable microenvironment for the microorganisms.They are also an important source of microbial biopesticides In this paper,the Odontotermes formosanus nest was used as a material.Multiple strains of Bacillus with antifungal activity were isolated from termite nests.The antifungal activity and mechanism of Bacillus YC-9 against Fusarium oxysporum f.sp.cucumerinum(FOC)as well as the preparation of wettable powder were studied.The results are described as follows:1.Seven different Bacillus strains with antagonistic activity to FOC were isolated from the O.formosanus nest.A Bacillus strain YC-9,which had a good antagonistic effect on FOC,was selected for further study.YC-9 was identified as Bacillus siamensis.The strain YC-9 had a good role of antagonism to FOC on plate with the inhibition rate of 72.5%,and also had good inhibition activites against other plant pathogenic fungi.The inhibition rates against Colletotrichum graminicola and Botrytis cinerea were reached 81.8%and 84.3%,respectively.Furthermore,the control effect of YC-9 against FOC was 73.2%in pot experiments.In addition,YC-9 had a growth promoting effect on the growth of cucumber.With the effect of YC-9,the promoting rates of fresh weight,dry weight,plant height and root length of cucumber seedlings were 42.58%,53.04%,20.81%and 19.26%,respectively.2.The mechanism of YC-9 against FOC was preliminarily discussed.Through the microscopic observation,YC-9 affected the mycelial morphology and spore formation of FOC.Experiments showed that YC-9 could produce extracellular enzymes(protease,pectinase,cellulase).It is preliminarily determined that one of the mechanisms of YC-9inhibiting FOC was the bacteriolysis through these hydrolases.These extracellular hydrolases could be produced by YC-9 solid fermentation,and the production of hydrolases was optimized.It was found that in most cases,the enzyme production at moderate humidity(substrate:water=1:1)was higher,and it was observed that alkaline treatment superiorly affected most of the studied substrates among acid/alkaline treatment,ultrasound and microwave assisted methods.There was a peak of peroxidase(POD),polyphenol oxidase(PPO)and phenylalanine ammonia-lyase(PAL)activity related to the resistance of cucumber roots on the 4th day of challenge inoculation with YC-9,which was3.06,2.81 and 1.84 times respectively,higher than that of the blank control cucumber seedlings,wich suggested that inoculating YC-9 could significantly improve the defensive enzyme activity of cucumber roots.YC-9 has the synthetic gene of antibiotic bacillomycin D detected by PCR with specific primers.The gene may express to produce bacillomycin D to inhibit FOC.In addition,YC-9 could still reach 4.7×10~7 CFU/g on the 30th day in the rhizosphere soil of cucumber,which could compete with pathogens for space sites and play a role of biological control in the soil.3.YC-9 was used to make wettable powder.The spore number of wettable powder was 3.33×10~6 CFU/g.Through optimization,it was found that the suspension rate of the wettable powder made of sodium lignosulfonate was good with the suspension rate of 78%.