Microbiology, pathogenesis, and epidemiology of anthrax
- Kenneth H Wilson, MD
Kenneth H Wilson, MD
- Professor of Medicine, Division of Infectious Diseases
- Duke University Medical Center
Bacillus anthracis, which causes anthrax, was the first clearly recognized bacterial pathogen. The life cycle of the organism was unraveled by Koch, who recognized the importance of dormant anthrax spores in the perpetuation of the organism in soil. These studies eventually led to Koch's postulates that have been the cornerstone for establishing a specific pathogen as the causative agent of human and animal diseases. Pasteur created the first successful antibacterial vaccine by successfully attenuating strains of B. anthracis and then proving that these strains could protect sheep from infection with fully virulent strains.
The microbiology, pathogenesis, and epidemiology of anthrax will be reviewed here. The clinical manifestations, diagnosis, treatment, and prevention of anthrax are discussed separately. (See "Clinical manifestations and diagnosis of anthrax" and "Treatment of anthrax" and "Prevention of anthrax".)
B. anthracis is a sporulating gram-positive rod (picture 1). It is nonmotile and grows rapidly at 37ºC on blood agar plates under aerobic conditions. Individual colonies are nonhemolytic and sticky. A gamma bacteriophage can be used to confirm the identity of the organism, and polymerase chain reaction (PCR) techniques can be used to identify as few as three spores of B. anthracis in a single specimen. All virulent strains are pathogenic in mice.
Virulent B. anthracis requires a poly-D-glutamic acid capsule and three proteins (edema factor, lethal factor, and protective antigen) that associate into two protein exotoxins as described below. Toxin and capsule production are dependent upon the presence of two plasmids:
●pX01 (184.5 kbp) is required for the production of the three exotoxins
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