Orthologues of Bacillus subtilis spore crust proteins have a structural role in the Bacillus megaterium QM B1551 spore exosporium.

The exosporium of Bacillus megaterium QM B1551 spores is morphologically distinct to exosporia observed in spores of many other species. Previous work has demonstrated that unidentified genes encoded on one of the large indigenous plasmids are required for the assembly of the Bacillus megaterium exosporium. Here we provide evidence that pBM600-encoded orthologues of the Bacillus subtilis CotW and CotX proteins, which form the crust layer in spores of that species, are structural components of the Bacillus megaterium QM B1551 spore exosporium. The introduction of plasmid-borne cotW and orthologous cotX genes to the PV361 strain, which lacks all indigenous plasmids and produces spores that are devoid of an exosporium, results in the development of spores with a rudimentary exosporium-type structure. Additionally, purified recombinant CotW protein is shown to assemble at the air-water interface to form thin sheets of material, which is consistent with the idea that this protein may form a basal layer in the Bacillus megaterium QM B1551 exosporium. Importance When starved of nutrients some bacterial species develop metabolically dormant spores that can persist in a viable state in the environment for several years. The outermost layers of spores are of particular interest since (a) these represent the primary site for interaction with the environment, and (b) the protein constituents may have biotechnological applications. The outermost layer, or exosporium, in Bacillus megaterium QM B1551 spores, is of interest as it is morphologically distinct to the exosporium of spores of the pathogenic Bacillus cereus family. In this work we provide evidence that structurally important protein constituents of the Bacillus megaterium exosporium are different to those in the Bacillus cereus family. We show also that one of these proteins can assemble when purified to form sheets of exosporium-like material. This is significant as it indicates that spore-forming bacteria employ different proteins, and mechanisms of assembly, to construct their external layers.