A previously healthy 46-year-old woman comes to the physician with a one-week history of productive cough and fatigue. Two weeks ago, she had fever, nasal congestion, rhinorrhea, and myalgias that resolved with supportive care. She has not traveled out of the United States. Pulmonary examination shows dullness to percussion and increased fremitus at the right middle lobe. An x-ray of the chest is shown. A sputum sample is most likely to show which of the following findings?

Gram-positive, alpha-hemolytic diplococci

Septate, acute-branching hyphae

Silver-staining, gram-negative bacilli

Encapsulated, gram-negative coccobacilli

Gram-positive, catalase-positive cocci

A 34-year-old woman presents with confusion, drowsiness, and headache. The patient’s husband says her symptoms began 2 days ago and have progressively worsened with an acute deterioration of her mental status 2 hours ago. The patient describes the headaches as severe, localized to the frontal and periorbital regions, and worse in the morning. Review of symptoms is significant for a mild, low-grade fever, fatigue, and nausea for the past week. Past medical history is significant for HIV infection for which she is not currently receiving therapy. Her CD4+ T cell count last month was 250/mm3. The blood pressure is 140/85 mm Hg, the pulse rate is 90/min, and the temperature is 37.7°C (100.0°F). On physical examination, the patient is conscious but drowsy. Papilledema is present. No pain is elicited with extension of the leg at the knee joint. The remainder of the physical examination is negative. Laboratory findings, including panculture, are ordered. A noncontrast CT scan of the head is negative and is followed by a lumbar puncture. CSF analysis is significant for: Opening pressure 250 mm H2O (70-180 mm H2O) Glucose 30 mg/dL (40-70 mg/dL) Protein 100 mg/dL (<40 mg/dL) Cell count 20/mm3 (0-5/mm3) Which of the following additional findings would most likely be found in this patient?

CSF India ink stain shows encapsulated yeast cells

Gram-positive diplococci are present on microscopy

CSF shows a positive acid-fast bacillus stain

Multiple ring-enhancing lesions are seen on a CT scan

CSF shows gram negative diplococci

While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?

Hemolytic reaction when grown on sheep blood agar

A 24-year-old man presents with a painless genital ulcer for the past 2 weeks. He reports that he recently has been having unprotected sex with multiple partners. Past medical history is unremarkable. On physical examination, a single ulcer is present on the dorsal shaft of the penis which is circumscribed, indurated, and partially healed. There is moderate inguinal lymphadenopathy but no buboes. Which of the following tests would confirm the most likely diagnosis in this patient?

Perform a darkfield microscopic examination of a swab from the chancre

Swab the chancre and perform a saline wet mount

Fluorescent treponemal antibody absorption (FTA-ABS) test

Venereal Disease Research Laboratory (VDRL) test

Klebsiella species for USMLE Step 3: High-Yield Microbiology Lessons

Klebsiella ID - The Encapsulated Offender

Microscopy & Culture: Gram-negative, enteric bacillus. Forms large, mucoid, glistening colonies on agar due to its prominent polysaccharide capsule.
Biochemical Markers:
- Lactose fermenter (pink colonies on MacConkey agar).
- Urease positive.
- Indole negative (helps differentiate from E. coli).
Key Virulence Factor: A thick, anti-phagocytic polysaccharide capsule is its hallmark. This results in a positive "string test."

⭐ The extensive capsule leads to the production of thick, mucoid, blood-tinged sputum, famously known as "currant jelly" sputum, in pneumonia patients.

Klebsiella pneumoniae mucoid colonies on blood agar

Virulence Factors - Slimy & Dangerous

Polysaccharide Capsule: The primary virulence factor; a thick, mucoid layer.
- Prevents phagocytosis and complement-mediated killing.
- Gives colonies their characteristic slimy, mucoid appearance.
- Hypervirulent strains (hvKp) exhibit a hypermucoviscous phenotype.
LPS (Endotoxin): A core component of the Gram-negative outer membrane.
- Triggers a potent inflammatory cascade, leading to fever and septic shock.
Adhesins (Fimbriae): Mediate binding to host epithelial cells, essential for colonization, particularly in the urinary and respiratory tracts.
Siderophores: High-affinity iron-chelating compounds (e.g., enterobactin) that sequester host iron, which is vital for bacterial growth.

⭐ The extensive capsule is responsible for the thick, mucoid, blood-tinged sputum classically described as "currant jelly" sputum in severe pneumonia.

Klebsiella pneumoniae capsule with India ink stain

Clinical Disease - The 3 A's

Aspiration Pneumonia:
- Classic in debilitated hosts, especially Alcoholics and Diabetics.
- Produces thick, mucoid, blood-tinged sputum ("currant jelly").
- CXR: Lobar consolidation, often in posterior upper lobes, with potential for cavitary lesions or abscess. The "bulging fissure" sign is a classic finding.
Abscesses:
- High propensity for abscess formation due to its thick capsule.
- Most notably causes pyogenic liver abscess, often with metastatic spread.
- Also a cause of lung, brain, and splenic abscesses.
UTIs & Bacteremia:
- Major cause of nosocomial UTIs, particularly in catheterized patients.
- Frequently leads to bacteremia, sepsis, and septic shock, especially in hospitalized patients.

⭐ Klebsiella granulomatis (formerly Calymmatobacterium granulomatis) causes Granuloma Inguinale (Donovanosis), an STI with painless, "beefy-red" genital ulcers. Diagnosis relies on identifying Donovan bodies-bacteria within macrophages-on tissue smear.

High‑Yield Points - ⚡ Biggest Takeaways

Klebsiella is a gram-negative, lactose-fermenting rod with a prominent polysaccharide capsule.

The thick mucoid capsule is a key virulence factor, leading to a mucoid look on agar.

Classically causes lobar pneumonia in alcoholics and diabetics, often with abscess formation.

Produces thick, bloody sputum famously described as "currant jelly" sputum.

A major cause of nosocomial UTIs, especially in catheterized patients.

Remember the 3 A's: Aspiration pneumonia, Abscesses, and Alcoholics.

Carbapenem-resistant Klebsiella pneumoniae (KPC) is a significant public health threat.

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