Morphogenesis of the Bacillus anthracis spore

Department of Microbiology and Immunology, Loyola University Medical Center, Maywood, IL 60153, U.S.A.; Bacteriology Division, United States Army Medical Research Institute of Infectious Diseases, Fort Detrick, Frederick, MD, 21702-5011, U.S.A.; Department of Biological, Chemical, and Physical Sciences, Illinois Institute of Technology, Chicago, IL 60616, U.S.A.; Deptartment of Molecular, Cellular and Developmental Biology, University of Michigan, Ann Arbor, MI 48109, U.S.A.; Headquarters, United States Army Medical Research Institute of Infectious Diseases, Fort Detrick, Frederick, MD, 21702-5011, U.S.A.

^* To whom correspondence should be addressed. Email: adriks{at}lumc.edu.

	Abstract

Bacillus spp. and Clostridium spp. form a specialized cell type called a spore, during a multistep differentiation process initiated in response to starvation. Spores are protected by a morphologically complex protein coat. The Bacillus anthracis coat is of particular interest because the spore is the infective particle for anthrax. We determined the roles in spore assembly and virulence of several B. anthracis orthologues of Bacillus subtilis coat protein genes. One of these, cotE, has a striking function in B. anthracis; it guides assembly of the exosporium, an outer structure encasing B. anthracis, but not B. subtilis spores. However, CotE has only a modest role in coat protein assembly, in contrast to the B. subtilis orthologue. cotE mutant spores are fully virulent in animal models, indicating that the exosporium is dispensable for infection, at least in the context of a cotE mutation. This has implications both for the pathophysiology of the disease as well as next generation therapeutics. CotH, which directs assembly of an important subset of coat proteins in B. subtilis, also directs coat protein deposition in B. anthracis. Additionally, however, in B. anthracis CotH effects germination; in its absence more spores germinate than in wild type. We also found that SpoIVA has a critical role in directing assembly of the coat and exosporium to an area around the forespore. This function is very similar to that of the B. subtilis orthologue, which directs assembly of the coat to the forespore. These results show that while B. anthracis and B. subtilis rely on a core of conserved morphogenetic proteins to guide coat formation, these proteins may also be important for species-specific differences in coat morphology. We further hypothesize that variation in conserved morphogenetic coat proteins may play roles in taxonomic variation among species.