[Identification and characterization of the outermost layer of Bacillus subtilis spores]

The Gram-positive bacterium Bacillus subtilis forms spores when conditions are unsuitable for growth. The spores are encased in a multilayered shell that includes a cortex and a spore coat, and remain viable for long periods in the harsh environment. In the present article, recent progress in our understanding of the outer structure of B. subtilis spores is reviewed in the Japanese language. Although spore coat assembly involves the deposition of at least 70 distinct protein species, the positions of most of such proteins have not been experimentally determined. To this end, the diameters of the protein layers and spores were measured using fluorescence microscopy and then the positions of proteins in the spore coat of B. subtilis spores were estimated. The locations of 16 proteins were determined using this method. One protein was assigned to the cortex, nine to the inner coat, and four to the outer coat. Further, two proteins, CgeA and CotZ, were assigned to a previously unidentified outermost layer. McKenney et al. have also identified the outermost layer using a similar method; the layer was termed the "crust". Immunofluorescence microscopy revealed that the crust is indeed the most external layer of B. subtilis spores. Mutational analysis indicated that all genes in the cotVWXYZ cluster were involved in spore crust synthesis and that CotY and CotZ played critical roles in crust formation.