Bacillus anthracis, Bacillus cereus, and Bacillus thuringiensis are the major pathogens of the spore-forming genus Bacillus and possess an outer spore layer, the exosporium, not found in many of the nonpathogenic species. The exosporium consists of a basal layer with the ExsY, CotY, and BxpB proteins being the major structural components and an exterior nap layer containing the BclA glycoprotein. During the assembly process, the nascent exosporium basal layer is attached to the spore coat by a protein linker that includes the CotO and CotE proteins. Using transmission electron microscopy, Western blotting, immunofluorescence, and fluorescent fusion protein approaches, we examined the impact of single, double, and triple mutants of the major exosporium proteins on exosporium protein content and distribution. Plasmid-based expression of exsY and cotE resulted in increased production of exosporium lacking spores, and the former also resulted in outer spore coat disruptions. The exosporium bottlecap produced by exsY null spores was found to be more stable than previously reported, and its spore association was partially dependent on CotE. Deletion mutants of five putative spore genes (bas1131, bas1142, bas1143, bas2277, and bas3594) were created and shown not to have obvious effects on spore morphology or BclA and BxpB content. The BclC collagen-like glycoprotein was found to be present in the spore and possibly localized to the interspace region. IMPORTANCE B. anthracis is an important zoonotic animal pathogen causing sporadic outbreaks of anthrax worldwide. Spores are the infectious form of the bacterium and can persist in soil for prolonged periods of time. The outermost B. anthracis spore layer is the exosporium, a protein shell that is the site of interactions with both the soil and with the innate immune system of infected hosts. Although much is known regarding the sporulation process among members of the genus Bacillus, significant gaps in our understanding of the exosporium assembly process exist. This study provides evidence for the properties of key exosporium basal layer structural proteins. The results of this work will guide future studies on exosporium protein-protein interactions during the assembly process.
Keywords: Bacillus anthracis; CotE; assembly; electron microscopy; endospores; exosporium; mutants.