Bacillus anthracis

Bacillus anthracis is a large, gram-positive, aerobic, spore-forming rod and the causative agent of anthrax. Its spores survive for years in soil, so infection classically occurs through contact with livestock, animal hides, or wool. Under the microscope the organism grows in long chains, sometimes described as a "serpentine" or boxcar-like appearance, and it is nonmotile — a distinguishing feature among Bacillus species.

Two virulence factors dominate testing. First, B. anthracis has the notable distinction of a protein capsule composed of poly-D-glutamate — nearly all other encapsulated bacteria use polysaccharide capsules. Second, it produces anthrax toxin, a three-component exotoxin: protective antigen (the binding component that lets the others enter cells), edema factor (an adenylate cyclase that raises cAMP and causes tissue swelling), and lethal factor (a protease that kills cells by disrupting MAP kinase signaling).

Clinical anthrax takes three forms. Cutaneous anthrax, the most common, begins as a papule that ulcerates into a painless black eschar with surrounding edema. Inhalational anthrax ("woolsorters' disease") follows spore inhalation and progresses from flu-like symptoms to hemorrhagic mediastinitis, classically showing a widened mediastinum on chest imaging, with high mortality. Gastrointestinal anthrax follows ingestion of contaminated meat. The organism is also a recognized bioterrorism agent.

The USMLE Step 1 exam tests B. anthracis among the gram-positive bacilli. High-yield associations include the poly-D-glutamate protein capsule, the edema factor–lethal factor–protective antigen toxin, the painless black eschar of cutaneous disease, and the widened mediastinum of inhalational anthrax.