Bacillus species

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Bacillus anthracis

• CAR bacillus in URT infection in cattle and URT infection in rabbits

• Can cause vascular fragility, leading to haemorrhagic disease.
• Causes severe systemic disease that can result in enteritis.
• peritonitis in pigs

Overview

• Mostly non-pathogenic environmental organisms
• Bacillus anthracis causes anthrax
• Anthrax is a severe disease affecting all mammals worldwide
• Ruminants are highly susceptible to anthrax, dying of a septicaemic form
• Pigs and horses are moderately susceptible, but carnivores are fairly resistant
• Bacillus licheniformis may cause shoradic abortions in cattle and sheep
• Bacillus cereus causes food poisoning in humans and mastitis in cows

Characteristics

• Large, Gram positive rods
• Produce endospores
• Aerobes or facultative anaerobes
• Grow on non-enriched media
• Motile
• Catalase positive, oxidase negative
• Bacillus anthracis colonies are up to 5mm diameter, flat, dry, grey, with a ground-glass appearance; curled outgrowthsfrom the edge of the colony give a 'medusa head' appearance; non-haemolytic (differentiate from Bacillus cereus)
• Bacillus licheniformis forms dull, rough, wrinkled colonies, with hair-like outgrowths
• Biochemical tests for identification
• Can often tolerate adverse environmental conditions

Bacillus anthracis

• Pathogenesis and pathogenicity:
  • Saprophyte in soil
  • 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 factor and lethal factor
  • Oedema factor is an adenylate cyclase which increases intracellular cAMP concetrations, damaging phagocytes
  • Protective factor binds to the cell receptor to allow action of the other factors
  • The toxin kills phagocytes, increases capillary permeability and interferes with clooting cascade
  • Capillary thrombosis; leakage of fluid through damaged capillary endothelium
  • Systemic shock from circulatory collapse, haemorrhage and oedema lead to death of the animal
• Clinical signs:

Bacillus licheniformis

• Widespread in the environment
• Associated with food spoilage
• Abortion in cattle and sheep, possibly from spoiled silage or hay