The crystalline layers in spores of Bacillus cereus and Bacillus thuringiensis studied by freeze-etching and high resolution electron microscopy.

The comparative morphology of spores from Bacillus cereus, B. thuringiensis S-9 (wildtype), B. thuringiensis HB 9-1 (acrystaliferous mutant) and B. finitimus, was studied by the freeze-etching technique. Particular attention was given to the three crystalline layers found in these spores, the pitted layer, the parasporal layer, and the basal layer of the exosporium with lattice constants of 9.2 nm, 5.8 nm and 8.0 nm respectively. The parasporal layers, corresponding to the fraction of Fl of Scherrer and Somerville [18] were found to be membraneous sheets laying between spore and exosporium. Analysis of the isolated exosporium by negative staining and digital image processing revealed a lattice structure belonging to the p 6 hexagonal space group with a repeat of 8.0 nm. The unit cell contains 6 subunits, each 3 nm in diameter, which are centered around a hypothetical channel readily penetrated by the stain.