Lecture 4: Aerobic Gram-Positive Spore-Forming Rods (Bacillus spp.)

Question 1

What are the key features of Bacillus spp.?

Answer 1

Gram-positive rods, aerobic, spore-forming, catalase-positive.

Question 2

How can B. cereus and B. anthracis be differentiated in the lab?

Answer 2

B. cereus: Beta-hemolytic (BA), motile, penicillin-resistant.

B. anthracis: Gamma-hemolytic (BA), non-motile, penicillin-sensitive.

Question 3

Compare the emetic and diarrheal forms of B. cereus food poisoning.

Answer 3

Emetic: Cereulide toxin (heat-stable), rapid onset (0.5–6 hrs), vomiting.

Diarrheal: Enterotoxins (Nhe, Hbl, CytK), delayed onset (6–15 hrs), diarrhea.

Question 4

How does cereulide toxin cause systemic damage?

Answer 4

Binds 5-HT3 receptors → mitochondrial dysfunction (K+ disruption) → liver/kidney damage.

Question 5

Name the three main enterotoxins in diarrheal B. cereus and their effects.

Answer 5

Nhe: Pore-forming → apoptosis.

Hbl: Pore-forming → pyroptosis (via NLRP3 inflammasome).

CytK: β-PFT → caspase-8 activation → macrophage death.

Question 6

How does B. cereus appear on PEMBA agar?

Answer 6

Blue colonies (no mannitol fermentation) with egg yolk inhibition (lecithinase-positive).

Question 7

Describe the endospore stain results for Bacillus.

Answer 7

Green spores (malachite green), pink vegetative cells (safranin).

Question 8

What are the two forms of anthrax, and how are they contracted?

Answer 8

Cutaneous (95%): Soil/farm animal contact.

Pulmonary (5%): Bioterrorism (inhalation of spores).

Question 9

How can B. cereus emetic toxin be prevented in food?

Answer 9

Refrigerate/freeze cooked rice promptly; avoid >2 hrs in “danger zone” (4–60°C).

Question 10

What tests confirm B. cereus infection?

Answer 10

Gram stain (Gram+ rods), spore stain (+), PEMBA (lecithinase+/mannitol−), beta-hemolysis (BA).