FIG. 6.

Examination of wild-type B. subtilis 168 and divIB mutant sporangia by transmission electron microscopy. (A to D) Various stages of engulfment in wild-type B. subtilis strain 168; (E to H) various stages of engulfment seen in the divIB mutant. In wild-type cells the first step of engulfment is thinning of the layer of peptidoglycan of the septal cell wall, at the center of the polar septum, allowing the forespore compartment to push into the mother cell (A and B). Once the peptidoglycan has been completely removed, the mother cell membrane migrates around the forespore (C) until the forespore is fully enclosed within the mother cell cytoplasm (D) (33). In the divIB mutant, thinning of the peptidoglycan of the polar septal wall sometimes started at the correct location, in the middle of the polar septum (E). However, sometimes septal wall thinning was initiated at the edges of the polar septum (F). The cell wall of the polar septum in the divIB mutant was incompletely thinned, resulting in a constricted bulging of the forespore into the mother cell (G and H). Bar in panel H, 200 nm.