Bacillus anthracis

Categorization
Cell Wall: Gram Positive Shape: Spore-forming Rod
Metabolism: Facultative Anaerobe
Virulence Factors
  • Capsule:
    • Uniquely, the anti-phagocytic bacillus capsule is not composed of polysaccharides but rather by the amino acid D-glutamine.
  • Exotoxins:
    • B. anthracis possesses a two potent A-B Toxins where the B "Binding Subunit" promotes entry of the A "Active Subunit" inside host cells
    • Protective Antigen (PA): Is the common B Subunit for both toxins
    • Edema Factor (EF): Is an A Subunit with adenylate cyclase activity that increases intracellular cAMP and induces cells to secrete large amounts of fluid
    • Lethal Factor (LF): Is an A Subunit with protease activity that deranges inflammatory signaling
Transmission
  • Exposure
    • Anthrax spores are extremely resilient and can survive for decades. Human infections occur following inoculation with spores either by their deposition in wounded skin, inhalation, or ingestion. However, once infected, humans cannot transmit the organism to one another.
  • Reservoirs
    • The natural reservoirs of anthrax are farm animals like cattle, goat, and sheep. Humans can become exposed after handling infected animal products such as pelts/skins or eating contaminated meats. Recently, Anthrax spores have been used as a bioterrorist weapon.
Clinical Consequences
  • Overview
    • The pathology that develops principally depends on the anatomic site of spore germination. Consequently, different syndromes result based on the mode of transmission
  • Gastrointestinal Anthrax
    • Anthrax spores germinate in the GI system following ingestion of infected meat. Exotoxin-mediated necrosis of GI mucosa results in a bloody infectious diarrhea. GI Anthrax is highly rare but is also highly fatal.
  • Cutaneous Anthrax
    • Anthrax spores germinate in the skin following inoculation of wounds. Characterized by a painless lesion with black necrotic center surrounded by a striking ring of edema. This is the most common form of anthrax infection and is fairly treatable. However, if untreated, bacteria can disseminate into bloodstream and become rapidly fatal.
  • Inhalational Anthrax
    • Anthax spores germinate in lung following inhalation of infected animal skins (AKA Woolsorter's Disease) or due to bioterrorism. Inhaled spores are rapidly phagocytosed by alveolar macrophages and transported to mediastinal lymph nodes where they germinate. Germination results in hemorrhagic necrosis of these lymph Nodes, and thus a hemothorax characterized by a hemorrhagic mediastinum as well as pleural effusions. On chest radiography this is observed as "Mediastinal Widening". Subsequent hematogenous dissemination is rapidly fatal.
Treatment
  • Rapid treatment with antibiotics is critical (esp. Penicillin V, ciprofloxacin, or doxycycline).