Sporeless mutants of Bacillus thuringiensis. III. The process of crystal formation

In order to investigate the formation of the parasporal crystal of B. thuringiensis with special reference to the spore, sequential ultrastructural analysis of sporulation was performed using a sporeless mutant strain (sp^?) as well as its parent wild strain (sp⁺). From the logarithmic growth to the end of forespore formation, the same sequential process of sporulation proceeded in both strains and a forespore with double membranes appeared. Thereafter, subsequent sporulation in the sp strain was either partly or completely arrested and finally spore (mainly the forespore) became deformed. On the other hand, crystal formation took place throughout by the same processes both in sp⁺ and sp^? strains. During the forespore formation, a primordial crystal and an ovoid inclusion appeared and after this stage, the crystal displayed a characteristic diamond-shaped body with lattice fringes increasing its size. No regularity was found in the position of the crystal with respect to the spore. As far as the present ultrastructural observations were concerned, the crystal developed without any special association with the membranes of the spore. However, without the formation of the forespore (including the incipient forespore), no crystal formation was observed.