Isolation And Identification Of An Antagonistic Bacterium Against Cucumber Wilt Disease And Study Of Its Relevant Properties

The cucumber wilt disease, a soil-transmitted botanic disease caused by one species affliated to the Fusarium oxysporum genus with a significantly negative impact on the production of cucumber, has a strong impact on both the yield and the quality of this product ascribed to its frequent outbreak in melon-producing regions. Originally, it's the conventional method to sprinkle the chemical pesticides in the fanning lands. However, various side effects, for instance, the leftover and pest resistance to these chemicals, have posed a major threat to diet security and the environment as well. Thus there has given rise to the biological prevention, due to its higher security, improved effects, an even more mild impact on the environment and its facilitating the sustainable development of organic agriculture and the betterment of ecology.An antagonistic bacterium, named as YHJ15, was isolated from soils sampled in vegetable greenhouses and planting areas in Shandong, Yunnan, Shanxi province. This bacterium was identified to be Bacillus pumilus sp. on the basis of its morphological, physio-chemical characteristics and its 16S rDNA sequence. With a preference to a moderately acidic habitat within the scope of pH5-7, this strain also demanded for a relatively large amount of oxygen for growth and had a wide tolerance range NaCl. The most optimal carbon and nitrogen sources were galactose and beef extract respectively. But this microbe was sensitive to antibiotics especially lincomycin. No remarkable differences of its growth velocity was determined within the incubation temperature range of 15-35℃. In addition, YHJ15 was capable of producing chitinase etc.A significant inhibitive effect of the fermentation solution was displayed on the pathogen of the cucumber wilt disease by means of confrontation, which fell insignicantly after the sterilization of the solution under 121℃for 30min. The inhibition by volatile products of YHJ15 was measurable to some extent to the pathogen but indicatting merely a relative inhibition ratio of 25.5%(as compared to the control defined as 100%); however, that by the non-volatile ones was a bit higher reaching the figure 48.9%. No restraint was measurable to budding ratio of the seeds of cucumber. An optimization test for highest yield of the antagonistic compounds was conducted, from which we know that the optimal carbon and nitrogen source source was glucose and soybean peptone respectively; the optimal pH range was 7-8 and the most appropriate inoculation rate was 2.5%; the ratio of 30mL/250mL was most appropriate for ventilation; the most proper culturing phase was 35h and the most temperature scope was 25-35℃.In this paper, a preliminary study of physio-chemical properties of the antagonistic compounds was also carried out. The effect of crude antagonistic coumpounds reached culmination at the the 70% saturization ratio of ammonium sulphate. When exposed to a temperature span of 50-100℃for 30min the materials suffered a little activity downwards. These chemicals was moderately acidic-stable but sensitive to alkalinity, representing a highest antagonistic activity in pH 5-8 but nearly down to zero when exposed to pH above 12. However, the digestion of proteases triggered some activity loss over 50%. And those harbored a relatively good sensitivity to ultraviolet, meanwhile, no strict correlation between the deduction range and the intensity and radiation span of ultraviolet was measuable. The materials were insensitive to chlorine. When laid under room temperature and 4℃repectively, the relative antagonistic effect of those fell down gradually along with the time. When under room temperature some sediments could been seen on the 14th day with the relative activity down to 34.27%, and no measurable activity on the 6th month; however, a remarkable deduction was observed till the 4th month when under 4℃. The concentrations of NaCl in the range of 0.2-1.0mol/L posed no distinct restraint on the activity. In addition, the crude solution containing antagonistic compounds would suffer a great activity loss after the sterilization, holding a relative activity level down to 37.29%, which indicated that partial components were thermally sensitive and the rest otherwise.