الفهرس 
Bacillus spp. as a promising biocontrol agentOnly 14 pages are availabe for public view
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Abstract
Biological control is one of the most promising eco-friendly methods used to protect plants from disease. Understanding the environmental factors that influence on the productivity of bioactive compounds by biocontrol agents is the key step toward improving the efficiency of their biocontrol activity. Therefore, the present study aimed at investigation biological control possibilities of cucumber damping-off caused by Pythium sp. using cellulase producing Bacillus sp. The most important results were:
1- Forty cellulolytic bacteria isolates belong to the genus Bacillus were obtained from the rhizosphere of healthy plants. The isolate B71 had the highest extracellular cellulase activity at 0.05 U/ml which identified as Bacillus subtilis, based on 16S rRNA analysis.
2- Twelve variables were screened using plackett-Burman design to determine the most influence factors affecting the production of cellulase enzyme. The results showed that CMC, peptone, temperature, NaCl and K2HPO4 were the most important variables that positively affected cellulase production. On the other hand, urea, pH and incubation period were the main factors that negatively affected enzyme activity.
3- A three-level Box-Behnken design was applied to complete the optimization where the most five effecting variables employed for response surface methodology (RSM). Statistical analysis of the Box-Behnken design results indicated that the optimal levels of the five variables were 10 g/l, CMC; 15 g/l, peptone; 39 °C, temperature; 0.5 g/l, NaCl and 1.0 g/l, K2HPO.
4- The statistical optimization resulted in a 2.56 fold increase in cellulase production in the optimized medium as compared to un-optimized medium.
5- The effect of agitation and aeration rates on cellulase activity in submerge fermentation showed that the maximum enzyme activity and bacterial biomass was achieved at the agitation speed of 300 rpm and aeration rate of 1.0 vvm. Cellulase productivity in batch fermentation was higher by approximately 3.8 times than in un-optimized culture.
6- Controlling medium pH during batch fermentation process resulted in a marked decrease in cellulase and bacterial biomass production as compared to un-controlled-pH.
7- The partial purified cellulase showed a final specific activity of 0.35 U/mg protein and was purified 5 fold after ammonium sulfate precipitation followed by dialysis. Zymographic analysis of the partially purified enzyme showed the presence of multiple cellulase system (55, 50 and 45 kDa).
8- Of the four Pythium isolates recovered from diseased cucumber seedling, only one isolate was highly pathogenic and caused 100 % pre- and post-emergence damping-off. These isolate was identified to the species level based on morphological characters along with sequences of the internal transcribed spacer (ITS) of the ribosomal DNA and showed 99% similarity to G. spinosum (basionym: Pythium spinosum).
9- Biocontrol activity of Bacillus subtilis was investigated against G. spinosum in vitro. The bacterial isolate showed a high antagonistic activity toward G. spinosum where the percentage of inhibition was up to 50% as compared to control.