Bacillus anthracis

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Anthrax (human). Courtesy of T. Scase
  • Epidemiology:
    • Saprophyte in soil
    • Endospore formation allows persistence and spread
    • Endospores survive decades in the soil
    • Outbreaks in herbivores grazing pastures contaminated by spores from buried carcases
    • Infection usually by ingestion of spores and penetration through damaged mucosa
  • Pathogenesis and pathogenicity:
    • Spores germinate at site of entry and spread via lymphatics to bloodstream, where they multiply and produce toxin
    • Capsule and toxin (encoded by separate plasmids) required for virulence
    • Capsule composed of homopolymer of D-isomer of glutamic acid allows survival in the body by resisting phagocytosis
    • Capsule stains mauve with polychrome methylene blue in the MacFadyean's reaction - identify anthrax in blood samples
    • Bacilli appear as chains of dark blue, square-ended rods surounded with the capsule
    • Extracellular toxin (holotoxin) composed of oedema factor, protective antigen and lethal factor
    • Oedema factor is an adenylate cyclase which increases intracellular cAMP concetrations, causing fluid accumulation and damaging neutrophils
    • Lethal factor causes release of cytokines from macrophages
    • Protective antigen binds to the cell receptor to allow action of the other factors
    • The toxin kills phagocytes, increases capillary permeability and interferes with clotting cascade
    • Capillary thrombosis; leakage of fluid through damaged capillary endothelium
    • Systemic shock from circulatory collapse, haemorrhagic disease and oedema lead to death of the animal
    • Severe systemic disease that can result in enteritis
    • Causes tissues to darken and swell due to oedema and necrosis

Anthrax