Bacillus species

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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 outgrowth sfrom 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

  • 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
  • Clinical signs:
    • Cattle/sheep:
      • Fatal peracute septicaemia
      • Animals usually found dead
      • Pyrexia, depression, congested mucous membranes and petechiae before death
      • Abortion, subcutaneous oedema and dysentry in animals surviving more that one day
    • Pigs:
      • Subacute anthrax with oedematous swelling of throat, head and regional lymph nodes
      • Intestinal form with high mortality - dysentry due to haemorrhagic enteric lesions
      • Peritonitis
    • Horses:
      • Subacute anthrax with subcutaneous oedema of thorax, abdomen and legs following entrance of spores into abrasions
      • Septicaemia with colic and dysentry due to haemorrhagic enteritis from ingestion of spores; ecchymoses and splenomegaly
    • Dogs
      • Rarely affected, but similar disease to that foung in pigs
    • Humans
      • Cutaneous anthrax - localised lesion from entrance into abrasion which can cause septicaemia
      • Pulmonary anthrax - inhalation of spores
      • Intestinal anthrax - ingestion of infective material
  • Diagnosis:
    • Post mortem: bloat, incomplete rigor mortis, ecchymoses, oedema, dark unclotted blood from orifices, blooy fluid in body cavities, splenomegaly


Bacillus licheniformis

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


Bacillus cereus

  • Mastitis in cattle
  • Food poisoning and eye infections in humans