A 71-year-old woman presents with high-grade fever and chills, difficulty breathing, and a productive cough with rust-colored sputum. She complains of a sharp left-sided chest pain. Physical examination reveals increased fremitus, dullness to percussion, and bronchial breath sounds on the lower left side. A chest X-ray shows left lower lobe consolidation. The offending organism that was cultured from the sputum was catalase-negative and had a positive Quellung reaction. The organism will show which gram stain results?

Cannot be seen with gram staining since the organism lacks a cell wall

Gram-positive cocci in clusters

A 29-year-old woman comes to the military physician because of a 2-day history of fever, joint pain, dry cough, chest pain, and a painful red rash on her lower legs. Two weeks ago, she returned from military training in Southern California. She appears ill. Her temperature is 39°C (102.1°F). Physical examination shows diffuse inspiratory crackles over all lung fields and multiple tender erythematous nodules over the anterior aspect of both legs. A biopsy specimen of this patient's lungs is most likely to show which of the following?

Spherules filled with endospores

Septate hyphae with acute-angle branching

Encapsulated yeast with narrow-based budding

Oval, budding yeast with pseudohyphae

An 11-year-old boy presents with a sore throat, fever, chills, and difficulty swallowing for the past 3 days. The patient’s mother says that last night he was short of breath and had a headache. Past medical history is unremarkable. The patient has not been vaccinated as his mother thinks it is "unnecessary". His temperature is 38.3°C (101.0°F), blood pressure is 120/70 mm Hg, pulse is 110/min, and respiratory rate is 18/min. On physical examination, the patient is ill-appearing and dehydrated. A grayish-white membrane and pharyngeal erythema are present in the oropharynx. Significant cervical lymphadenopathy is also present. A throat swab is taken and gram staining shows gram-positive club-shaped bacilli along with few neutrophils. Which of the following would most likely be the result of the bacterial culture of the throat swab in this patient?

Small black colonies on tellurite agar

Hemolytic black colonies on blood agar

Metallic green colonies on eosin-methylene blue agar

Greyish-white colonies on Thayer-Martin agar

Creamy white colonies on Loeffler's serum

Bacillus species for USMLE Step 2 CK: High-Yield Microbiology Lessons

Bacillus Basics - Spore-forming Rods

Gram-positive rods, often in chains.
Aerobic and catalase-positive.
Spore-forming: Highly resilient spores allow survival in harsh conditions (heat, desiccation). Ubiquitous in soil.
Key species:
- B. anthracis (anthrax)
- B. cereus (food poisoning)

⭐ The capsule of B. anthracis is unique, made of poly-D-glutamate protein, protecting it from phagocytosis.

Bacillus with green endospores

B. anthracis - The Anthrax Agent

Microbiology: Gram ⊕, spore-forming rod in chains ("boxcar" appearance). Aerobic, non-motile. Unique polypeptide capsule (poly-D-glutamic acid).
Virulence Factors:
- Anthrax Toxin (Exotoxin): Composed of three proteins.
  - Protective Antigen (PA): Mediates entry of other factors.
  - Edema Factor (EF): Adenylate cyclase → ↑cAMP → massive edema.
  - Lethal Factor (LF): Zinc metalloprotease → cleaves MAP kinase → necrosis & apoptosis.
Clinical Manifestations:
- Cutaneous (most common): Painless papule → ulcer with a central black eschar.
- Inhalational/Pulmonary: Spores inhaled → hemorrhagic mediastinitis. Often fatal.

⭐ A widened mediastinum on chest X-ray is a classic finding in inhalational anthrax.

Chest X-ray: Widened mediastinum in inhalational anthrax

Treatment: Ciprofloxacin or Doxycycline.

B. cereus - Fried Rice Fiend

Pathogen: Aerobic, motile, spore-forming, Gram-positive rod.
Source: Ubiquitous in soil; contaminates food.
Toxin-Mediated Illness (2 types):
- Emetic (vomiting): Preformed, heat-stable toxin (Cereulide).
  - Vehicle: Reheated fried rice (classic). 📌 Mnemonic: "Be serious! Reheating rice is vomit-inducing!"
  - Onset: Rapid (1-6 hrs).
- Diarrheal: Heat-labile enterotoxin produced in vivo.
  - Vehicle: Meats, vegetables, sauces.
  - Onset: Slower (8-16 hrs).

⭐ High-Yield: The emetic toxin is pre-formed and heat-stable. Reheating contaminated rice will kill the B. cereus bacteria but will NOT inactivate the toxin, leading to rapid-onset illness.

Lab & Rx - Identifying & Treating

Lab ID:
- Large, Gram-positive rods in chains; spore-forming (aerobic).
- B. anthracis: Non-hemolytic, "medusa head" colonies on blood agar.
- B. cereus: β-hemolytic on blood agar.
Treatment (Rx):
- Anthrax (B. anthracis):
  - Cutaneous: Ciprofloxacin or Doxycycline.
  - Inhalational/Systemic: Ciprofloxacin + Clindamycin. Consider Raxibacumab (antitoxin).
- Food Poisoning (B. cereus):
  - Supportive care. Vancomycin for severe non-GI infections.

⭐ High-Yield: The capsule of B. anthracis is uniquely composed of a polypeptide (poly-D-glutamic acid), unlike the polysaccharide capsules of most other bacteria.

High‑Yield Points - ⚡ Biggest Takeaways

Bacillus anthracis causes anthrax via its unique poly-D-glutamate capsule and anthrax toxin.

Cutaneous anthrax presents with a painless black eschar; pulmonary anthrax shows a widened mediastinum on chest X-ray.

Bacillus cereus is a common cause of food poisoning, classically from reheated rice (emetic toxin).

Both are large, Gram-positive, spore-forming rods that grow in aerobic conditions.

Spores are highly resilient and central to transmission.

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