Physiological Formation of Mosquitocidal Toxin by a novel Bacillus thuringiensis isolate under Solid State Fermentation

Abstract: Sixty eight cultures were isolated from soil of different locations in El-Sharkia governorate that were cultivated by cereals, rice, clover, cotton, and maize crops. Based on the LC50 and LC90 values, the Egyptian isolate No 4 was selected for further study due to its lower LC50 and LC90 than the international Bacillus thuringiensis var.israelensis (Bti) upon bioassay against second instars larvae Culex pipiens. The Egyptian isolate No 4 is defined morphologically and biochemically as Bacillus thuringiensis. Physiological factors affecting growth and toxin biosynthesis in B. thuringiensis isolate No 4 in comparison to Bti under solid state fermentation were studied. Talcum powder and silica gel were the best carriers yielding highest mosquitocidal activity for Bacillus thuringiensis No.4 and Bacillus thuringiensis var.israelensis, respectively.The highest mortality were obtained upon utilization of kidney beans (3%) and black eyed beans (6%) as base solid substrate for B. thuringiensis isolate No 4 and B. thuringiensis var.israelensis, respectively. Toxicity increased with extended the incubation period up to 9 days for both of tested organisms. Highest values for growth and toxicity were obtained in cultures with initial moisture content adjusted at 60 % (w/w) for both organisms. Mosquitocidal toxin activity were fairly stable within a wide range of increasing aeration level up to a ratio of 10g culture/250ml conical flask and 10g culture/500ml for growth and toxicity o