An exosporium-like outer layer in Bacillus subtilis spores

BACTERIAL endospores are differentiated cells surrounded by several characteristic spore envelopes—cortex, inner and outer spore coats. In addition to these layers, present in all Bacillus and Clostridium spores, several species possess a loosely fitting outermost structure—the exosporium1—5. In other species, however, including B. subtilis, the exosporium has not been found in thin sections1,6,7, although its possible existence was suggested by the observation of a fibrous ‘exosporial’ layer by freeze-etching8. In our studies of the fine structure of B. subtilis spores we have found that a new outer layer, probably an exosporium, is present and can be visualised in thin sections after partial chemical extraction of the spore coats.