Structural analysis of Bacillus subtilis spore peptidoglycan during sporulation

A major structural element of bacterial endospores is a peptidoglycan (PG) wall. This wall is produced between the two opposed membranes surrounding t he developing forespore and is composed of two layers. The inner layer is t he germ cell wall, which appears to have a structure similar to that of the vegetative cell wall and which serves as the initial cell wall following s pore germination. The outer layer, the cortex, has a modified structure, is required for maintenance of spore dehydration, and is degraded during spor e germination. Theories suggest that the spore PG may also play a mechanica l role in the attainment of spore dehydration. Inherent in one of these mod els is the production of a gradient of cross-linking across the span of the spore PG. We report analyses of the structure of PG found within immature, developing Bacillus subtilis forespores. The germ cell wall PG is synthesi zed first, followed by the cortex PG. The germ cell wall is relatively high ly cross-linked. The degree of PG cross-linking drops rapidly during synthe sis of the first layers of cortex PG and then increases two- to eightfold a cross the span of the outer 70% of the cortex. Analyses of forespore PG syn thesis in mutant strains reveal that some strains that lack this gradient o f cross-linking are able to achieve normal spore core dehydration. We concl ude that spore PG with cross-linking within a broad range is able to mainta in, and possibly to participate in, spore core dehydration. Our data indica te that the degree of spore PG cross-linking may have a more direct impact on the rate of spore germination and outgrowth.