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Mother cell lysis (Stage VII)

7. Mother cell lysis (Stage VII)

The sporulation process ends approximately eight hours after it began with release of the mature spore by lysis of the mother cell (applies for standard laboratory conditions at 37°C). The forespore moves from the pole to the middle of the cell, a process also known as detachment ¹⁶⁷. Shortly thereafter the cytoplasmic membrane of the mother cell ruptures, followed by lysis of the mother cell wall attained by cell wall amidases CwlC and CwlH ¹⁶⁸. Genes encoding these autolysins are under the control of σ^K and GerE in the mother cell and are the latest to be expressed. Premature lysis is prevented by an as of yet unidentified factor that is required for activation of the autolysins ³⁶.

After mother cell lysis, proteins and chromosomal DNA are released into the environment. The latter is degraded by DNase NucB¹⁶⁷, which is produced under the control of σ^E and secreted during earlier stages of sporulation¹⁶⁹. Nutrients in the form of nucleotides and peptides can serve as energy sources for surrounding cells that have not yet committed to sporulation. In microcolonies this phenomenon has been observed where a subpopulation of cells started to commit to sporulation, whereas surrounding cells lysed and the released nutrients allowed for a second round of growth of neighbouring cells ¹⁷⁰. This process has been described as either cannibalism or altruism ^{45, 171}.  

Once released, the spore exists as a dormant entity that is highly resistant against heat, acids, desiccation and other extreme environmental conditions ¹⁷². Due to the very low amounts of water and the high quantity of DPA in the spore core, it was long assumed that spores are metabolically inactive. Recent reports, however, show that post-release enzymatic reactions are required for full maturation of coat proteins and acquisition of heat resistance ¹⁷³ and changes in RNA content were observed up to several days after completion of sporulation ¹⁷⁴. It therefore appears that spores undergo a so-called 'adaptive period' shortly after release from the mother cell before adopting the dormant state.