However, the residual virulence of pXO1 − strains appears to be strongly influenced by the backgrounds of both pXO2 and the chromosome ( 32, 35). anthracis wild-type strains of either plasmid attenuates virulence. Two large virulence plasmids, pXO1 and pXO2, encode toxin production and the formation of a poly-γ- d-glutamic acid capsule, respectively ( 20, 30). PA is the common cell binding component and is required for toxin activity ( 14). The bacilli secrete three proteins, protective antigen (PA), lethal factor (LF), and edema factor (EF), and these proteins combine to form the lethal (PA plus LF) and edema (PA plus EF) toxins. Toxemia and septicemia rapidly lead to death. After entry into the host, the spores germinate and yield toxin-producing, capsulated bacilli. The disease anthrax is caused by Bacillus anthracis, a gram-positive spore-forming bacterium. Guinea pigs and mice had very different susceptibilities to infection with the nontoxigenic strain, highlighting the importance of verifying the pertinence of animal models for evaluating anthrax vaccines. This suggests that spore antigens contribute to protection. Immunization with FIS alone was sufficient to protect mice partially, and guinea pigs totally, against infection with this strain. A PA-deficient challenge strain was constructed, and its virulence was due solely to its multiplication. The toxin-neutralizing activities of sera from mice immunized with PA alone or PA plus FIS were similar, suggesting that the protection conferred by PA plus FIS was not only a consequence of the humoral response to PA. anthracis strains in mice and guinea pigs. anthracis to PA elicits total protection against challenge with virulent B. Here we demonstrate that the addition of formaldehyde-inactivated spores (FIS) of B. However, in experimental animals such vaccines are less protective than live attenuated strains. Currently used acellular vaccines against anthrax consist of protective antigen (PA), one of the anthrax toxin components. Septicemia and toxemia rapidly lead to death in infected mammal hosts. Anthrax is caused by Bacillus anthracis, a gram-positive spore-forming bacterium.
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