Bacillus anthracis

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

• Causes severe systemic disease that can result in enteritis.
• Peritonitis in pigs