Bacillus anthracis

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.