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Bacillus anthracis

Gram-positive, spore-forming, encapsulated, non-motile rod that causes anthrax, manifesting as cutaneous, inhalational, or gastrointestinal disease.

Organism Card

DomainMust know
Identity
  • G+ve, large, square-ended rod; forms central/subterminal spores [1]
  • Aerobic / facultatively anaerobic; non-motile (vs B. cereus which is motile)
  • Genus Bacillus; forms poly-D-glutamic acid capsule (anti-phagocytic)
  • Classical example of infectious disease dominated by virulence of the microorganism [2]
Lab discriminator
  • Bacillus in blood cultures is most often a contaminant — but B. anthracis is the exception [1]
  • Non-haemolytic on blood agar (vs B. cereus → β-haemolytic)
  • Non-motile (vs B. cereus → motile) — key discriminator
  • "Medusa head" / ground-glass colonies on agar; "string of pearls" test +
  • Capsule visualised with India ink or M'Fadyean polychrome methylene blue stain
Reservoir / transmission
  • Widely distributed in environment, e.g. soil [1]
  • Spores survive decades in soil; reservoir = herbivores (cattle, sheep, goats)
  • Routes: cutaneous (contact with hides/wool), inhalational (woolsorter's disease), GI (undercooked contaminated meat)
  • Bioterrorism agent (Category A); spore-containing letters / powder
Key virulence
  • Anthrax toxin = 3-component A-B toxin:
  • Protective antigen (PA) — binds host cell, mediates entry
  • Lethal factor (LF) — metalloprotease → macrophage lysis
  • Edema factor (EF) — adenylate cyclase → massive oedema
  • Poly-D-glutamic acid capsule → anti-phagocytic; encoded on pXO2 plasmid
  • Toxin genes on pXO1 plasmid
Clinical syndromes
  • Cutaneous anthrax (~95%): painless papule → vesicle → black eschar ("malignant pustule"); surrounding non-pitting oedema; low mortality if treated
  • Inhalational anthrax (woolsorter's disease): widened mediastinum on CXR (haemorrhagic mediastinitis); biphasic illness → rapid septic shock; ~high mortality
  • GI anthrax: bloody diarrhoea, ascites, sepsis after contaminated meat
  • Injection anthrax: soft tissue oedema in IVDU (reported in Europe)
Diagnosis
  • Specimen: skin lesion swab/vesicle fluid (cutaneous); blood cultures; sputum/BAL (inhalational)
  • Gram stain: large G+ve rods in chains ("bamboo stick" / "boxcar" appearance)
  • Culture on blood agar → non-haemolytic, ground-glass colonies
  • PCR for pXO1/pXO2 targets; DFA (direct fluorescent antibody)
  • CXR clue: widened mediastinum ± pleural effusions (inhalational)
Treatment
  • Cutaneous: ciprofloxacin or doxycycline (oral); 7–10 days
  • Inhalational/systemic: IV ciprofloxacin + another agent (e.g. clindamycin, meropenem); add antitoxin (raxibacumab/obiltoxaximab) if available
  • Penicillin historically used but concern for β-lactamase (some strains carry cephalosporinase)
  • Duration 60 days post-exposure (spore germination concern)
Prevention
  • Notifiable disease in Hong Kong
  • Anthrax Vaccine Adsorbed (AVA / BioThrax) — PA-based; used for military / high-risk occupational exposure
  • Post-exposure prophylaxis: ciprofloxacin or doxycycline × 60 days ± vaccine
  • Decontamination: sporicidal agents; autoclaving; animal carcass disposal (burn/bury in quicklime)
  • Bioterrorism preparedness: Category A CDC select agent
Classic traps
  • B. anthracis vs B. cereus: anthracis = non-motile, non-haemolytic, capsulated; cereus = motile, β-haemolytic, food poisoning (emetic/diarrhoeal)
  • Bacillus spp. in blood culture usually = contaminant; do NOT dismiss if clinical suspicion for anthrax [1]
  • "Black eschar" + painless + occupational animal exposure → think cutaneous anthrax (not spider bite / MRSA abscess)
  • Widened mediastinum DDx: anthrax vs aortic dissection vs lymphoma — anthrax has fever + prodrome

Exam Intelligence

  1. "Bacillus — widely distributed in environment, e.g. soil; B. anthracis: anthrax; B. cereus: food poisoning" — GC 105 slide directly lists these as the two examinable Bacillus species [1]. Expect MCQs asking you to differentiate anthrax from cereus.

  2. "Bacillus in blood cultures is most often a contaminant" — GC 105 emphasises this as an expert practice tip [1]. The exam implication: if a stem describes Bacillus in blood culture, the default answer is "contaminant" unless the clinical context (e.g. bioterrorism exposure, black eschar, widened mediastinum) specifically points to B. anthracis.

  3. "Anthrax (Bacillus anthracis) & Rabies are classical examples of infectious disease dominated by the virulence of the microorganisms" — GC 102 handout uses anthrax as the paradigm for virulence-driven disease at one extreme, versus opportunistic infections (S. epidermidis, Candida, CMV) at the other extreme in immunocompromised hosts [2]. This framing could appear as an SAQ concept question.

  4. Bacillus spp. listed as a cause of catheter-related bacteraemia in GC 102 handout device-infection table [2]. In an immunocompromised host with an indwelling vascular catheter, Bacillus spp. bacteraemia may be real (not contaminant) — a nuance worth noting for minicase stems.

  5. GC 105 groups B. anthracis under "Medically important Gram-positive rods" alongside Listeria monocytogenes [1]. A classic EMQ discriminator: Listeria = cephalosporin-resistant, treat with ampicillin; Bacillus anthracis = treat with ciprofloxacin/doxycycline.

  • Non-motile is the single most tested lab feature separating B. anthracis from other Bacillus spp. (especially B. cereus).
  • Non-haemolytic on blood agar — another key discriminator vs B. cereus (β-haemolytic).
  • The 3-component toxin (PA + LF + EF) is a favourite MCQ target: PA is the binding component; LF = lethal (metalloprotease); EF = edema (adenylate cyclase). PA is also the vaccine antigen.
  • Widened mediastinum on CXR in a patient with flu-like prodrome + rapid deterioration → inhalational anthrax. Trap: do not confuse with aortic dissection (no tearing back pain, different demographics).
  • Black eschar = anthrax (Greek anthrakis = coal). Painless lesion — if the stem says "painful", consider other diagnoses (e.g. MRSA abscess, ecthyma gangrenosum).
  • pXO1 (toxin) and pXO2 (capsule) — both plasmids needed for full virulence. Vaccine strains lack pXO2 (Sterne strain) or are PA subunit-based.

No past paper questions directly testing Bacillus anthracis were identified in the indexed Fourth Summative MCQ, SAQ, or Minicase papers (2016–2025). The organism may appear as a distractor option or within bioterrorism/infection-control themed questions not captured in the retrieved excerpts. Students should still know anthrax for EMQ organism-matching and MCQ discriminator questions, given its explicit mention in GC 105 and GC 102 lecture material.

[1] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf [2] Lecture slides: GC 102. Fever after chemotherapy infections in immunocompromised hosts [Handout].pdf

Enterococcus faecium

Gram-positive, catalase-negative coccus found in chains, notable for intrinsic vancomycin resistance (VRE) and as a leading cause of nosocomial urinary tract infections, bacteremia, and endocarditis.

Bacillus cereus

Gram-positive, spore-forming, aerobic rod that produces emetic and diarrheal enterotoxins, commonly causing food poisoning associated with reheated rice and starchy foods.

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