Bacillus anthracis and Anthrax
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Direct answer
Bacillus anthracis is a large, spore-forming, non-motile Gram-positive rod that causes anthrax through a poly-D-glutamate capsule and a tripartite toxin. Protective antigen carries lethal factor and oedema factor into host cells. Cutaneous anthrax, the most common natural form, produces a painless black eschar, while inhalational anthrax is the most lethal form and presents with a biphasic illness and a widened mediastinum.
What you must remember
- Morphology: large square-ended Gram-positive rods in chains, with a non-immunogenic capsule of poly-D-glutamic acid and spores that resist disinfection.
- Colonies: non-haemolytic, non-motile colonies with a Medusa-head or curled appearance on blood agar, and the organism is penicillin susceptible.
- Toxin components: protective antigen binds host receptors, lethal factor is a zinc metalloprotease that cleaves MAP kinase kinases, and oedema factor is an adenylate cyclase.
- Cutaneous anthrax: a painless papule that becomes a black eschar surrounded by oedema, the commonest natural form.
- Inhalational anthrax: an initial nonspecific phase followed by sudden respiratory failure, with haemorrhagic mediastinitis and a widened mediastinum on chest X-ray.
- Gastrointestinal anthrax: from undercooked meat, with abdominal pain, haematemesis and bloody diarrhoea.
- Biodefence: Category A bioterrorism agent in the CDC classification, with inhalational exposure the main concern.
Pathogenesis, step by step
Anthrax spores enter the body through broken skin, the lungs or the gut, and the disease unfolds in a fixed sequence. The steps show why the toxin, rather than the bacterium alone, drives death.
- Spore entry: spores enter through a wound, by inhalation or by ingestion, and germinate in the host tissue.
- Capsule protection: the poly-D-glutamate capsule resists phagocytosis, so vegetative bacteria multiply in the lymph and blood.
- Toxin production: the bacteria secrete protective antigen, lethal factor and oedema factor, encoded on the pXO1 plasmid.
- Receptor binding: protective antigen is cleaved and assembles into a pore-forming complex that binds host receptors, carrying the other two factors into the cell.
- Cellular damage: lethal factor disrupts signalling pathways, while oedema factor raises intracellular cyclic AMP, causing fluid leakage and cell death.
- Clinical outcome: oedema and haemorrhagic necrosis at the entry site, or mediastinal bleeding in inhalational disease, lead to shock.
Where the exam tries to catch you
Anthrax questions often rely on one clinical clue, a colony feature or a toxin component.
- Trap: calling a painful furuncle cutaneous anthrax Correct: cutaneous anthrax is typically painless, with a black eschar and surrounding oedema.
- Trap: assuming a widened mediastinum on X-ray means pneumonia Correct: inhalational anthrax shows haemorrhagic mediastinitis, a key diagnostic clue after nonspecific early symptoms.
- Trap: equating a Medusa-head colony with Bacillus cereus Correct: B. cereus is typically beta-haemolytic, whereas B. anthracis is non-haemolytic and forms Medusa-head colonies.
Frequently asked questions
What are the clinical forms of anthrax?
The clinical forms of anthrax are cutaneous, inhalational and gastrointestinal, and injection anthrax is recognised in people who inject drugs. Cutaneous anthrax is the most common natural form, while inhalational anthrax carries the highest mortality.
Which toxin components does Bacillus anthracis produce?
Bacillus anthracis produces protective antigen, lethal factor and oedema factor, which together form the anthrax toxin. Protective antigen binds host receptors and delivers the other two components into cells, where they disrupt signalling and fluid balance.
What is the characteristic colony appearance of Bacillus anthracis?
Bacillus anthracis forms non-haemolytic colonies with a Medusa-head or curled appearance on blood agar. Together with non-motility and penicillin susceptibility, this feature helps distinguish it from related Bacillus species.
Which chest X-ray finding suggests inhalational anthrax?
A widened mediastinum from haemorrhagic mediastinitis on chest X-ray suggests inhalational anthrax, especially after an initial phase of nonspecific symptoms. Pleural effusion can accompany it, and the finding supports the diagnosis when the clinical picture fits.
Why is Bacillus anthracis classed as a bioterrorism agent?
Bacillus anthracis is classed as a bioterrorism agent because its spores can be aerosolised, causing inhalational disease with high mortality. The CDC places it in Category A because of its potential for mass casualties and the difficulty of early diagnosis.