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BTI

BTI
                           Bti is an acronym for Bacillus thuringiensis subsp. israelensis sertype H-14, a bacteria that produces protein crystals with larvacidal activity against mosquitoes. As diagrammed,the Bti bacteria is a rod-shaped organism which is 3 to 5 microns in length and 1.2 to 1.5microns in width. On sporulation, Bti forms an endospore and produces three parasporalbodies (protein crystals) bound by a fibrous envelope. Worldwide, there are 19 subspecies or varieties of B.thuringiensis.
 
Sporulated Bti cell
(Redrawn from Kurstak,1982)
 
 

 
 
MODE OF ACTION
 
                   The protein crystals formed by the Bti bacteria on sporulation are classified as a delta-endotoxin. In the crystalline state the delta-endotoxin is considered a protoxin because it must be activated by protease's (digestive juices) of the target oranisms to become a toxic substance. Once ingested by a mosquito, the alkaline conditions of the stomach dissolve the crystal and release the delta-endotoxin. The toxin has an affinity for the stomach wall lining causing the cells to first swell then rupture. When enough stomach cells burst, the alkaline fluid of the midgut can enter the blood. This movement of stomach fluid increases the alkalinity of the blood and results in a general paralysis.
 
LIFE CYCLE
                    As illustrated, the basic life cycle of bacteria involves two interlinked cycles: (1) vegetative growth and (2) sporulation. During the vegetative growth cycle bacteria reproduce by a process called binary fission, whereby a single cell divides into two cells. These daughter cells continue to divid until they enter the sporulation phase. In this cycle, a thickened wall structure develops within the cellwall of the bacteria. This structure is called an endospore and is the dormant stage of the bacteria.
 

 
ENVIRONMENTAL IMPACT
 
                  Unlike broad spectrum insecticides, Bti has a  selective action. Only mosquitoes, black flies and some midges are susceptible to the delta-endotoxin. Other aquatic invertebrates and non-target insects are unaffected by applications of Bti.

 

 
 
 

 

 

 
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