Studies onBacillus thuringiensis H-14 strains isolated in Egypt-II. Ultrastructure studies

Area of Science:

• Microbiology
• Insect Pathology
• Biochemistry

Background:

• Bacillus thuringiensis (Bt) H-14 is a key bacterium for mosquito control.
• Understanding the structure of its insecticidal inclusions is crucial for optimizing its efficacy.
• Previous studies have described various Bt crystal shapes, but novel forms and developmental pathways remain areas of investigation.

Purpose of the Study:

• To investigate the ultrastructure and development of insecticidal inclusions in two mosquito-toxic Bacillus thuringiensis H-14 isolates.
• To characterize the shapes, sizes, and subunit organization of parasporal crystals.
• To elucidate the initiation and assembly process of these insecticidal structures during sporulation.

Main Methods:

• Electron microscopy (thin sections and freeze-fracture) was employed to examine the morphology of inclusions.
• Ultrastructural analysis focused on sporulation and parasporal crystal development stages.
• Subunit size and organization within the crystals were analyzed.

Main Results:

• Inclusions displayed significant variation in shape, size, and stainability, with some previously unreported forms.
• Insecticidal inclusions initiated during axial filament formation in sporulation.
• A common, geometrically regular subunit formed first, followed by amorphous subunits, leading to diverse final inclusion shapes (cuboidal, rhomboid, etc.).
• Subunits measured approximately 5x5 nm in freeze-fractured crystals.

Conclusions:

• Bacillus thuringiensis H-14 parasporal crystal formation involves a complex, multi-stage process with a geometrically regular initial subunit.
• The observed diversity in inclusion morphology suggests varied developmental pathways and potential implications for insecticidal activity.
• This study provides novel ultrastructural details on Bacillus thuringiensis H-14 crystal development, contributing to the understanding of entomopathogenic bacteria.