Abstract
When examined with a phase-contrast microscope, all of the 15 Bacillus thuringiensis serovar yunnanensis (H20ab) strains, the reference type strain and the 14 Japanese isolates, produced large rhomboidal inclusions in spore-lacking cells but not in spore-forming cells. No envelopes were associated with inclusions. Inclusion matrix consisted of electron-dense homogeneous substances. The frequency of inclusion-forming cells was <1% of the cell population in each strain. Also, phase-dark spherical bodies, much smaller than rhomboidal inclusions, were often associated with cells in sporulating cultures. Electron microscopic studies with the four selected yunnanensis strains revealed that: (1) the majority of cells produced spores but not crystalline inclusions, (2) formation of crystalline inclusions was limited to the spore-lacking cells, and (3) electron-dense round bodies, much smaller than rhomboidal crystalline inclusions and putatively assigned to poly-β-hydroxybutyrate inclusions, were often observed in both spore-forming cells and inclusion-forming asporogenous cells. The overall results strongly suggest that the formation of crystalline inclusions in asporogenous cells is an H serotype-specific phenotype characteristic of the B. thuringiensis serovar yunnanensis.
