Bacillus anthracis coordinates the expression of its virulence factors in response to a specific environmental signal. Anthrax toxin proteins and the antiphagocytic capsule are produced in response to growth in increased atmospheric CO2. This CO2 signal is thought to be of physiological significance for a pathogen which invades mammalian host tissues. Immunity Considerable variation in genetic susceptibility to anthrax exists among animal species. This is illustrated in the table below.
Second attacks are extremely rare.
Permanent immunity to anthrax seems to require antibodies to both the toxin and the capsular polypeptide, but the relative importance of the two kinds of antibodies appears to vary widely in different animals. A nonencapsulated toxigenic strain has been used effectively in livestock. The Sterne Strain of Bacillus anthracis produces sublethal amounts of the toxin that induce formation of protective antibody. The best vaccine for humans is a preparation of the protective antigen of the lethal toxin recovered from culture filtrates of the bacteria. In either case, frequent boosters are necessary to maintain resistance to anthrax challenge.