Editors' ChoiceMicrobiology

Programmed death of bacterial spores

Sci. Signal.  06 Oct 2015:
Vol. 8, Issue 397, pp. ec283
DOI: 10.1126/scisignal.aad5579

Some bacteria, like Bacillus subtilis, form dormant, dessicated spores to survive periods of nutrient deprivation. During sporulation, the bacterium divides asymmetrically, and the smaller daughter—the forespore—is engulfed by the larger mother cell. The forespore acquires a cortex and a coat and undergoes additional modifications to produce a mature spore that is released into the environment when the mother cell lyses. Tan et al. found that CmpA, which is produced by the mother cell and localizes to the outer surface of the forespore, causes improperly formed B. subtilis spores to lyse. In cells overexpresing CmpA, the spore cortex formed, but the majority of spores lysed, thus greatly reducing the formation of viable spores. Mutations in genes encoding enzymes required for lysis of the mother cell did not rescue the production of viable spores in CmpA-overexpressing cells. CmpA interacted genetically with SpoIVA and SpoVM. SpoIVA is a component of the coat that is recruited to the outer surface of the forespore by SpoVM. Deletion of cmpA suppressed sporulation defects in B. subtilis strains carrying various mutant alleles of SpoVM that affect different aspects of spore envelope assembly, indicating that the absence of CmpA can prevent lysis of improperly formed spores. Biochemical experiments demonstrated that CmpA bound directly to ClpX, a subunit of the protease ClpXP, in both vegetatively growing and sporulating cells. In sporulating cells, CmpA bound to both ClpX and SpoIVA. Genetic experiments combined with fluorescence imaging showed that CmpA promoted the degradation of SpoIVA when the spore coat complex was not properly assembled, leading to spore lysis. Although it is not known how CmpA-mediated degradation of SpoIVA is limited to malformed spores, these results demonstrate that CmpA functions in a quality-control checkpoint that ensures that only properly formed spores are released into the environment (see Meneghini).

I. S. Tan, C. A. Weiss, D. L. Popham, K. S. Ramamurthi, A quality-control mechanism removes unfit cells from a population of sporulating bacteria. Dev. Cell 34, 682–693 (2015). [PubMed]

M. D. Meneghini, Spore no more: Quality control during bacterial development. Dev. Cell 34, 611–612 (2015). [PubMed]