Sporulation

Sporulation is a developmental process in which bacilli develop into heat-, chemical-, solvent-, radiation-, and desiccation-resistant spores (64,65). Their transition from dormancy to vegetative cells involves germination and outgrowth. Under laboratory conditions at 37 °C, sporulation takes approximately 8 h. The process has been divided into a series of stages that are defined primarily on the basis of their morphology. These stages, labeled 0, II, III, . . . through VII (stage I is no longer recognized), broadly correlate with the time (in hours) after the induction of sporulation (Fig. 3).

Sporulation is induced in response to nutrient deprivation and is only initiated after the failure of transition phase processes (e.g., motility and chemotaxis, production of macromolecular hydrolases, development of competence) to restore vegetative growth. Sporulation is induced by activated SpoOA (SpoOA-P) via a phosphorelay that facilitates multifactorial signal processing. The first morphologically distinct stage is stage II, which is defined by the formation of an asymmetric cell division event that divides the cell into two compartments, the larger sporangium (or mother cell) and the prespore, or forespore. During stage II, the prespore becomes completely engulfed by the sporangium so that, at stage III, the two compartments are separated by a double membrane system. During stage IV, peptidoglycan is laid down between the two membrane layers to form the spore cortex and the germ cell wall. During this stage the prespore becomes refractile to light. In stages V and VI, spore coat proteins are laid down on the surface of the inner and outer membrane of the prespore. As these mature the spore becomes increasingly resistant to chemicals and heat. In the final stage, stage VII, the by-now-dehydrated spore is released from the sporangium by cell lysis. The fully mature spore is metabolically inactive and capable of surviving as such for certainly tens, and possibly hundreds, of years.

Sporulation is a tightly controlled process requiring differential expression of genes on the chromosomes in each of the two compartments, as well as developmental checkpoints to ensure synchrony between their patterns of development. This is achieved through the differential activation of sporulation-specific sigma (o) factors at or about stages II to III. rF is active in the prespore, and rE is active in the sporangium. These sigma factors are, in turn, responsible for the induction of sporangium-specific (rK) and prespore-specific (rG) sigma factors. Synchrony between the two compartments is maintained by signal communication processes.

Much less is known about germination and outgrowth. In the laboratory at 37 °C, the time between induction of germination and the first cell division event is about 90 min. Germination is triggered by a variety of chemical and physical stimuli; spores of B. subtilis germinate in response to L-alanine or to a mixture of asparagine, glucose, fructose, and KCl (66). Germination is accompanied by a series of rapid chemical and physiological changes including the degradation of the spore cortex, disruption of the spore coat, rehydration, and swelling. These changes are accompanied by a loss of heat and chemical resistance. It takes some 30 min before de novo RNA synthesis takes place. During outgrowth the cell begins to increase in mass and to replicate its DNA, forming an elongated cylindrical cell that is only subsequently reduced to a normal-sized cell by a delayed cell division.

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