Figure 1. The sporulation pathway in B. subtilis.

Membranes are in red, and peptidoglycan in gray. Transcription factors active in different cells and stages are shown red. I. In predivisional cells, the division sites (dotted ovals) are shifted to polar positions. II. Polar septation produces a small forespore and a large mother cell. III. After polar septation, the mother cell engulfs the forespore in a phagocytosis-like process. After engulfment, the forespore is enclosed within the mother cell cytoplasm and delimited by two membranes. IV. The forespore matures through the formation of a peptidoglycan cortex (gray) between the inner and outer forespore membranes, the assembly of a proteinaceous coat around the outer membrane (black), and the dehydration of the forespore core, which accumulates dipicolinic acid (DPA). V. Once maturation is completed, the mother cell lyses and the spore is released into the environment, where it remains dormant until conditions are appropriate for germination. OM, outer membrane; OC, outer cortex; IC, inner cortex; IM, inner membrane.