Ultrastructure and development of an exosporium-like outer spore envelope in Bacillus subtilis

An exosporium-like outermost envelope is occasionally observed in thin sections of Bacillus subtilis spores. Treatment of the mature spores with urea and mercaptoethanol (sometimes completed by sodium dodecyl sulfate) or with NaOH, disorganizes and partially solubilizes the outer spore coat. This treatment permits a clear visualization of the exosporium in all spores of several B. subtilis strains. Exosporium appears either as a single sheet, 8-9 nm thick, or with a triple-layered unit membrane-like profile. Frequently it exhibits a crystal-like periodic pattern. The exosporium primordium appears first at stage IV, and its development is apparently independent of the formation of the cortex and of the spore coats. No morphological relationship was found between the outer forespore membrane and the exosporium.