Abstract：The expression of most insecticidal crystal proteins (ICPs) in Bacillus thuringiensis (Bt) is dependent on spore formation. In order to study the relationship between ICPs expression and spore formation at the cellular level, a pair of strains Bt 4.0718 and Bt ΔleuB were selected and subjected to FM464 staining at different growth stages for a laser confocal microscope observation. Results showed that the Bt 4.0718 strain underwent the asymmetric septum and forespore engulfment stages successively, and could enter into the next stage and successfully complete the entire process of spore development before the mother cell apoptosis. The asymmetric septum stage of Bt ΔleuB was significantly delayed, and the spore development was arrested in forespore engulfment. The crystals were visible since the asymmetric cell division stage, and the volume continued to increase until the mother cell died. qRTPCR result revealed that the high transcriptional level of σE, spoIIR and spoIIGA is a key factor to maintain the normal expression of ICPs in Bt ΔleuB cells. These results possessed a certain reference value for further revealing the relationship between sporulation and ICPs expression, and construction of engineering strains with better behavior.
