ABSTRACTBacillus anthracisspores are the infectious form of the organism for humans and animals. However, the approved human vaccine in the United States is derived from a vegetative culture filtrate of a toxigenic, nonencapsulated B. anthracisstrain that primarily contains protective antigen (PA). Immunization of mice with purified spore proteins and formalin-inactivated spores (FIS) from a nonencapsulated, nontoxigenic B. anthracisstrain confers protection against B. anthracischallenge when PA is also administered. To investigate the capacity of the spore particle to act as a vaccine without PA, we immunized mice subcutaneously with FIS from nontoxigenic, nonencapsulated B. cereusstrain G9241 pBCXO1-/pBC210-(dcG9241), dcG9241 ?bclA, or 569-UM20 or with exosporium isolated from dcG9241. FIS vaccination provided significant protection of mice from intraperitoneal or intranasal challenge with spores of the virulent B. anthracisAmes or Ames ?bclAstrain. Immunization with dcG9241 ?bclAFIS, which are devoid of the immunodominant spore protein BclA, provided greater protection from challenge with either Ames strain than did immunization with FIS from BclA-producing strains. In addition, we used prechallenge immune antisera to probe a panel of recombinant B. anthracisSterne spore proteins to identify novel immunogenic vaccine candidates. The antisera were variably reactive with BclA and with 10 other proteins, four of which were previously tested as vaccine candidates. Overall our data show that immunization with FIS from nontoxigenic, nonencapsulated B. cereusstrains provides moderate to high levels of protection of mice from B. anthracisAmes challenge and that neither PA nor BclA is required for this protection.
