The spore coat

The coat varies considerably in width among species. In Bacillus subtilis, where the coat is relatively wide, it is just less than 200 nm in width, and its multilayered organization is unmistakable by transmission electron microscopy (TEM). Importantly, the number of coat layers and the presence or absence of appendages extending from the coat surface vary among species. This interspecies variation and differences in complexity drew attention as soon as spores were imaged at high resolution, and in the decades since (1-7). The coat is readily distinguished from the cortex (see reference 178) because of its higher electron density. In a large subset of species, the spore also possesses an additional layer surrounding the coat, called the exosporium (Fig. 1; see also references 8 and 9).