Exosporium

The exosporium is the outer surface layer of mature spores. The exosporium represents the primary surface between the spore and its environment or host, and is a site of spore antigens.

Salt and detergent washing of exosporium fragments remove these and revealed proteins that are likely to represent structural or integral exosporium proteins. Seven proteins have been identified in washed exosporium: alanine racemase, inosine hydrolase, ExsF, CotY, ExsY, CotB, and a novel protein, ExsK. CotY, ExsY and CotB are homologues of Bacillus subtilis outer spore coat proteins, but ExsF and ExsK are specific to B. anthracis and other members of the Bacillus cereus group. Some exosporium material was attached to the spores. Whole spores were removed by low-speed centrifugation, the supernatant was filtered (0·45 μm or 0·2 μm) to remove all remaining live spores, and the exosporium fragments were concentrated.

The exosporium protein ΔywdL has a more fragile exosporium, subject to damage on repeated freeze-thawing. YwdL is important for formation of a robust exosporium, but is not required to maintain the crystalline assembly within the basal layer or for attachment of the hairy nap structure. ΔywdL spores are unable to germinate in response to CaDPA. This protein causes altered germination properties with a phenotype that confirms the expected defect in localization of the cortex lytic enzyme CwlJ in the coat.