BACILLUS

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BACILLUS

• Gram positive large rods
• Produce resistant spores
• Aerobes or facultative anaerobes
• Motile except B. anthracis
• Grow very fast
• Large colonies at 24 hours
• White-greyish, flat
• non-haemolytic (or slightly) on blood agar
• with irregular edges (Medusa-head appearance)
• Do not grow on MacConkey
• Biochemically active
• Catalase positive
• Majority non pathogenic environmental organisms

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Main species of BACILLUS

• B. anthracis - ANTHRAX in cattle, other species
  and man
• B. cereus food poisoning, eye, soft tissue
  infection (man) mastitis (cattle rare)
• B. licheniformis sporadic abortion in cattle
  and sheep
• B. subtilis
• B. stearothermophilus - Used in monitoring
  effectiveness of sterilisation (grow up to 75C)

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Anthrax

• Severe disease affecting virtually all mammalian
  species including man
• Worldwide, endemic or in defined regions
• Ruminants highly susceptible develop rapidly
  fatal septicaemia
• Pigs, horses moderately susceptible
• Carnivores comparatively resistant
• Birds almost totally resistant to infection

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Anthrax - Epidemiology

• Endospore formation most important factor in
  persistence of anthrax in environment and spread
  to other animals
• Endospores survive for decades in soil (up to 50
  years)
• Outbreaks can occur in herbivores when pasture is
  contaminated by spores from buried carcasses
• Spores bought to surface by flooding, excavation,
  subsidence, earthworm activity
• Infection acquired by ingestion of spores
  (inhalation, skin abrasion more common in man)

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Anthrax - Clinical

• Incubation hours days
• Cattle and sheep Septicaemic, rapidly fatal
• Most animals found dead

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Anthrax bacilli occluding the lumen of a blood
capillary. A characteristic of anthrax is the
massive bacteraemia in the last hours of life. At
the point of death, numbers often exceed 108
bacilli/ml blood. (Scanning electron micrograph)
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Anthrax - Diagnosis

• Carcasses are bloated, putrify and do not exhibit
  rigor mortis
• Bleeding issues from mouth, nostrils, anus
• If suspected from cattle and sheep, thin blood
  smear should be made from blood taken from tail
  or ear veins. In horses and pigs, smear can be
  made from oedematous fluid, e.g. peritoneal
  fluid.
• Blood collected from animal stained with
  polychrome methylene blue (or Giemsa) blue
  bacteria, pink capsules. McFadyeans capsule
  stain
• Inoculate agar plates for growth
• Characteristic colonial morphology, biochemical
  tests
• Mucoid colonies on bicarbonate agar, very
  susceptible to penecilline, inverted fir tree
  type of liquefaction on gelatine stab culture,
  and weak lecithinase activity on egg-yolk agar

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B. anthracis
polychrome methylene blue shows square ended,
blue rods, surrounded by pink capsular material
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Anthrax - Pathogenesis

• Virulence of B. anthracis derives from presence
  of poly-D-glutamic acid capsule and complex
  toxin.
• Both virulence factors encoded on plasmids and
  required for disease.
• Expression of these virulence factors regulated
  by host temperature and carbon dioxide
  concentration.
• Capsule inhibits phagocytosis.
• Toxin consists of 3 antigenic components
  (protective antigen, oedema factor, and lethal
  factor).
• PA acts as the binding moiety for both oedema
  factor and lethal factor.

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The mode of action of anthrax toxin. Protective
antigen (PA) binds to host cell-surface receptors
(1) and is cleaved by a cell-surface protease
releasing a 20-kDa piece (2). This exposes a
secondary receptor for the lethal factor (LF) and
edema factor (EF) components of the toxin, which
compete to bind to it (2). The complex is
internalized by receptor-mediated endocytosis (3)
and acidification of the resulting vesicle leads
to the transfer of the LF or EF across the
vesicular membrane into the cytosol where they
effect their catalytic actions (4).
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Anthrax - Pathogenesis

• Oedema factor is a calmodulin dependent adenylate
  cyclase and once inside cells following binding
  to PA increases cAMP causing the build up of
  fluid in tissue (tissue swells as seen in
  clinical disease)
• Oedema factor also inhibits neutrophil function,
  could be leukocidal
• LF stimulates macrophages to release cytokines
  IL-1 beta and TNF alpha
• Local effects are swelling, darkening of tissue,
  oedema and necrosis
• In septicaemia, leads to increased vascular
  permeability, haemorrhage, shock and death

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Signal for macrophage activation switched to cell
death. May explain why host overwhelmed
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Anthrax - Humans

• 3 forms of disease in man
• Cutaneous (malignant pustule) endospores enter
  abraded skin septicaemia if untreated
• Pulmonary woolsorters disease inhalation of
  spores
• Intestinal ingestion of infective material
• All fatal if not recognised early
• Bioterrorist weapon of choice!?

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Anthrax - Control

• Do not open carcass as this will facilitate spore
  formation and environmental contamination!
• Report to regulatory authority
• Annual vaccination (endemic regions)
• Chemoprophylaxis (in outbreaks)
• Carcasses incinerated or buried away from water
  courses
• Contaminated material disinfected in 10 formalin
  or if appropriate incinerated