Klebsiella pneumoniae is one of those “classic Step bugs” that shows up everywhere: pneumonia questions, UTIs, liver abscess vignettes, alcoholism/aspiration stems, and antimicrobial resistance. If you can recognize its thick capsule, mucoid lactose-fermenting colonies, and the clinical scenario of currant jelly sputum, you’ll pick up a lot of easy points on Step 1 (and Step 2).
What is Klebsiella pneumoniae?
Klebsiella pneumoniae is a Gram-negative, encapsulated, lactose-fermenting rod in the Enterobacterales family.
Quick ID card (memorize this)
- Gram stain: Gram-negative rods
- Oxygen: Facultative anaerobe
- Motility: Non-motile
- Oxidase: Oxidase-negative
- Key virulence factor: Thick polysaccharide capsule
- Culture: Lactose fermenter → pink colonies on MacConkey agar
- Colony appearance: Mucoid (capsule)
First Aid cross-reference: Microbiology → Gram-negative rods → Lactose fermenters (Klebsiella, E. coli, Enterobacter)
Where does it live (and why do we care)?
Klebsiella is part of the normal flora of the GI tract and can colonize the oropharynx—especially in hospitalized patients. Most disease happens when it:
- aspirates into the lungs (pneumonia), or
- enters the urinary tract (catheters), or
- translocates and seeds other sites (e.g., liver abscess)
High-yield risk settings:
- Alcohol use disorder
- Diabetes
- Hospitalization / ICU
- Intubation
- Aspiration risk
- Indwelling urinary catheters
Pathophysiology: Why it causes “currant jelly” pneumonia
The capsule is the star
Klebsiella’s large polysaccharide capsule is strongly anti-phagocytic and helps it evade host defenses.
What the capsule leads to:
- Mucoid colonies in culture (sticky, stringy appearance)
- Severe lobar pneumonia with necrosis and tissue destruction
- Thick inflammatory exudate in airways → classically described as “currant jelly” sputum (bloody, mucoid)
Classic Step mechanism link
- Alcoholism + aspiration → impaired airway protection → Klebsiella reaches lungs → lobar pneumonia, often severe.
First Aid cross-reference: Microbiology → Virulence factors (capsules), and clinical associations for Klebsiella (alcoholics, aspiration)
Clinical presentations you must recognize
1) Pneumonia (classic board favorite)
Typical vignette:
- Older patient with alcohol use disorder or aspiration risk
- Fever, cough, and “currant jelly” sputum
- May have upper lobe predominance (aspiration classically affects dependent lobes, but Klebsiella is often taught with upper lobes in alcoholics)
- Can cause lung abscesses due to necrosis
Imaging (high-yield clues):
- Lobar consolidation
- Bulging fissure sign (due to heavy exudate expanding the lobe)
2) UTI (especially nosocomial)
- Dysuria, frequency, fever
- Often tied to catheter-associated UTIs
- Consider Klebsiella among common Enterobacterales causes of UTIs
3) Pyogenic liver abscess (Step 2–leaning but shows up)
A particularly high-yield association: hypervirulent K. pneumoniae can cause:
- Liver abscess ± bacteremia
- Possible metastatic infection (e.g., endophthalmitis—more common in certain populations and clinical contexts)
Diagnosis: How Step expects you to identify it
Lab & culture hallmarks
- Gram-negative rods
- Lactose fermenter
- MacConkey agar: pink colonies
- Mucoid colonies (capsule)
- Non-motile (helps differentiate from some other enteric rods in questions)
Helpful “differential” table (Step-style)
| Feature | Klebsiella pneumoniae | E. coli | Pseudomonas aeruginosa |
|---|---|---|---|
| Gram stain | G− rod | G− rod | G− rod |
| Lactose fermentation | Yes | Yes | No |
| Capsule | Prominent | Variable | Not the classic feature |
| Motility | Non-motile | Motile | Motile |
| Classic clue | Currant jelly, bulging fissure | UTIs, neonatal meningitis (K1), HUS | Burn wounds, CF, ecthyma gangrenosum, grape odor |
| Oxidase | Negative | Negative | Positive |
Treatment (and the resistance traps Step loves)
Treatment depends heavily on:
- Site/severity (pneumonia vs UTI vs bacteremia)
- Susceptibility (ESBL? carbapenemase?)
Empiric thinking (board-style principles)
- For serious hospital-acquired pneumonia or sepsis, start broad and narrow when susceptibilities return.
- For community disease, narrower coverage may be enough depending on local patterns and risk factors.
Resistance patterns to know cold
ESBL producers (extended-spectrum beta-lactamases)
- Can inactivate many penicillins and cephalosporins.
- Carbapenems are commonly effective (classic exam answer when ESBL is emphasized).
Carbapenem-resistant Klebsiella (CRE)
This is the “nightmare” hospital pathogen category.
High-yield mechanism:
- Carbapenemase production (e.g., KPC)
High-yield treatment options (conceptual; stewardship/local susceptibility matters):
- Ceftazidime-avibactam (commonly tested as a KPC option)
- Meropenem-vaborbactam
- Sometimes polymyxins (older salvage therapy; toxicity)
Step takeaway: When you see ESBL → think carbapenem. When you see CRE/KPC → think newer beta-lactam/beta-lactamase inhibitor combos.
First Aid cross-reference: Pharmacology → Beta-lactams and beta-lactamase inhibitors; Microbiology → gram-negative rods and resistance mechanisms
High-yield associations (rapid review)
Must-remember buzzwords
- “Currant jelly sputum” → necrotizing pneumonia
- Alcohol use disorder → aspiration risk → Klebsiella pneumonia
- Bulging fissure on CXR
- Large polysaccharide capsule → mucoid colonies, anti-phagocytic
- Lactose fermenter → pink on MacConkey
- Nosocomial infections: pneumonia, UTI (catheters)
- ESBL / CRE resistance patterns
“If you see this, think Klebsiella” stems
- ICU patient on a ventilator + lactose-fermenting G− rod
- Alcoholic with lobar pneumonia + bloody mucoid sputum
- Culture described as mucoid, pink colonies on MacConkey
Common Step mistakes (avoid these)
- Mixing up lactose fermenters: If it’s pink on MacConkey, it’s likely E. coli, Klebsiella, or Enterobacter. Use capsule + currant jelly + non-motile to pick Klebsiella.
- Forgetting resistance: Klebsiella is a classic ESBL/CRE organism. When resistance language appears, the question is often really about drug choice.
- Confusing aspiration patterns: Aspiration pneumonia often involves anaerobes and dependent lobes, but boards frequently tie Klebsiella + alcoholism together—go with the test-writer’s pattern recognition.
Mini self-check (5 rapid-fire prompts)
-
Lactose fermenter on MacConkey → what color colonies?
Pink -
Key virulence factor causing mucoid colonies and anti-phagocytosis?
Polysaccharide capsule -
Alcohol use disorder + lobar pneumonia + bloody mucoid sputum = ?
Klebsiella pneumoniae -
ESBL-producing Klebsiella: classic go-to antibiotic class?
Carbapenems -
CRE/KPC concern: name a commonly used targeted combo.
Ceftazidime-avibactam (also meropenem-vaborbactam)