Microbiology · MU Sofia

73. Genus Bacillus - B. anthracis.

Covers Bacillus anthracis, beginning with its morphology, capsule and spore features, then staining and the distinctive colonial growth on culture media. It goes on to resistance, the three-component anthrax toxin with its plasmid-encoded virulence factors, and the zoonotic route of infection. The cutaneous, pulmonary and gastrointestinal clinical forms are described, followed by laboratory diagnosis, treatment, prevention and a closing comparison with anthracoid bacilli. A summary table of virulence factors is included.

Inside: 12 sections · 10 labelled pictures · 1 table · 16 flashcards · 4 min

Topic 73 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 12 sections, about 473 words (4 min of reading), with 16 flashcards.

In this note

  1. Genus Bacillus – Bacillus anthracis
  2. General Characteristics
  3. Microscopy & Staining
  4. Cultural Characteristics
  5. Resistance
  6. Virulence Factors
  7. Pathogenesis
  8. Clinical Forms of Anthrax
  9. Laboratory Diagnosis
  10. Treatment
  11. Prevention
  12. Anthrax vs. Anthracoid Bacilli (Pseudoanthrax)

General Characteristics

  • Gram-positive, large, rod-shaped bacilli
  • Spore-forming – spores are oval, central, non-bulging
  • Non-motile (unlike most other Bacillus species)
  • Arranged singly, in pairs, or in long chains ("bamboo stick appearance")
  • Aerobic or facultative anaerobe
  • Forms polypeptide capsule (unique – made of D-glutamic acid)
  • Capsule visible only in the presence of CO₂ or bicarbonate
  • Grows on simple media (not fastidious)

Microscopy & Staining

  • Gram stain: Large G⁺ rods in chains
  • Capsule: Detected using India ink or McFadyean’s stain (shows pink capsule on blue bacilli)
  • Spore stains: Peshkov, Sudan Black B
  • In culture, gives the “bamboo stick” appearance
  • No spores seen in clinical samples — only in culture

Cultural Characteristics

  • Growth at 37°C on blood or meat-peptone agar
  • Colonies:
  • Large, irregular, grey-white
  • “Medusa head” appearance (interlacing chains at colony edges)
  • Non-hemolytic (important feature)
  • Gelatin stab culture: Shows “Inverted fir tree” pattern
  • String of Pearls reaction
  • Selective media:
  • PLET medium (Polymyxin, Lysozyme, EDTA, Thallous acetate) — used to isolate B. anthracis

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