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Microbiology, pathogenesis, and epidemiology of anthrax

Kenneth H Wilson, MD
Section Editor
Daniel J Sexton, MD
Deputy Editor
Anna R Thorner, MD


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 helped to underpin Koch's postulates, a milestone in establishing specific pathogens as the causative agents 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 techniques can be used to identify as few as three spores of B. anthracis in a single specimen.

There is less genomic variation in B. anthracis than in any other known bacterial species that causes disease in humans. As a result, strain differentiation relies on demonstrating variations in copy number of tandem repeating DNA segments [1] or finding single-nucleotide polymorphisms by whole-genome sequencing [2]. B. anthracis is not phylogenetically distinct from Bacillus cereus and is presently considered a clade within the species B. cereus [3]. However, because of its medical significance, B. anthracis retains its original species name.


Portals of entry and dissemination — B. anthracis can invade the body by four routes: gastrointestinal, transcutaneous, pulmonary, and direct parenteral injection.

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Literature review current through: Nov 2017. | This topic last updated: Nov 29, 2017.
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