Immunization Studies with Attenuated Strains of Bacillus anthracis

Live, attenuated strains of Bacillus anthracis lacking either the capsule plasmid pX02, the toxin plasmid pX01, or both plasmids, were tested for their efficacy as vaccines against intravenous anthrax toxin challenge in Fischer 344 rats and against aerosol or intramuscular, virulent anthrax spore challenge in Hartley guinea pigs. Only those animals immunized with toxigenic, non-encapsulated (pX01)+, pX02- strains survived toxin or spore challenge and demonstrated post-immunization antibody titers to the three components of anthrax toxin (protective antigen, lethal factor, and edema factor). Immunization with two non-toxigenic, encapsulated (pX01-, pX02+), Pasteur vaccine strains neither provided protection nor elicited titers to any of the toxin components. Therefore to successfully immunize against anthrax toxin or spore challenge, attenuated, live strains of B. anthracis must produce toxin components specified by the pX01 plasmid.