Klebsiella pneumoniae

Classification & Microbiology

• Capsule (K antigen): Large polysaccharide capsule - the organism's primary virulence factor. Blocks complement deposition and opsonophagocytosis. Gives colonies a glistening, mucoid appearance on agar. On Gram stain, a visible negative space (halo) may be seen around the rods.
• Indole negative (unlike K. oxytoca, which is indole positive)
• Non-motile (unlike most Enterobacteriaceae)
• Ferments lactose (produces acid and gas)
• Urease and ONPG positive

Pathotypes

Virulence Factors

1. Polysaccharide capsule - antiphagocytic, anti-complement
2. Lipopolysaccharide (LPS/O antigen) - endotoxin activity, serum resistance
3. Fimbriae (type 1 and type 3) - adhesion to uroepithelium and respiratory mucosa
4. Siderophores (in hvKp: aerobactin iucA, salmochelin iroB) - iron acquisition, greatly enhance virulence
5. Rmp proteins (rmpA, rmpA2) - upregulate capsule production, confer hypermucoviscosity
6. Outer membrane proteins - contribute to antibiotic resistance

Clinical Syndromes

1. Pneumonia

• Classic population: Chronic alcoholics, malnourished patients, diabetics, COPD patients, debilitated elderly
• Classic features: Lobar consolidation (often upper lobe, right side), "currant jelly sputum" (thick, blood-tinged mucoid sputum due to abundant capsular polysaccharide), "bulging fissure sign" on CXR
• Modern presentations are often less dramatic - patchy infiltrate rather than massive lobar consolidation
• Complications: lung abscess, empyema, pulmonary necrosis, pleural effusion
• Hospital-acquired: ventilator-associated pneumonia (VAP) is a major concern

2. Urinary Tract Infection

• cKp causes only 1-2% of UTIs in healthy adults but 5-17% of complicated UTIs (structural/functional abnormalities, catheter use)
• hvKp more commonly presents as renal or prostatic abscess via bacteremic spread rather than ascending infection

3. Primary Liver Abscess (hvKp hallmark)

• Characteristic of the hypervirulent pathotype, especially in Southeast Asia
• Occurs in previously healthy, often younger individuals
• CT shows solitary right lobe abscess
• Metastatic spread to eye (endophthalmitis - can cause blindness), CNS, lungs, joints
• Often requires percutaneous drainage + prolonged antibiotics

4. Bacteremia / Sepsis

• Klebsiella bacteremia can arise from any infected site: urinary tract (15-30%), respiratory tract (15-30%), abdominal (hepatic abscess) (15-30%), intravascular device (5%)
• Associated with high mortality in MDR strains

5. Other Infections

• Neonatal sepsis and meningitis in LMICs
• Wound and soft tissue infections
• Rhinoscleroma (K. rhinoscleromatis) - chronic granulomatous nasal infection; Mikulicz cells
• Granuloma inguinale/Donovanosis (K. granulomatis)
• Pneumonia in severely malnourished children in low- and middle-income countries

Clinical Image - Hypervirulent K. pneumoniae

• Top: Abdominal CT of a healthy 24-year-old - primary liver abscess (red arrow) with metastatic spread to the spleen (black arrow)
• Middle: Endophthalmitis in a healthy 33-year-old
• Bottom: Positive string test - mucoid colony stretching >5 mm with inoculating loop, indicating hypermucoviscous phenotype

Laboratory Diagnosis

• Gram stain: Gram-negative rods, sometimes with visible capsular halo
• Grows readily on standard media (blood agar, MacConkey agar - pink/lactose-fermenting)
• Mucoid, glistening colonies; string test can be performed
• Indole negative (K. pneumoniae sensu stricto); Urease positive
• Definitive hvKp identification: PCR/genomic detection of all 5 biomarkers: iucA, iroB, peg-344, rmpA, rmpA2

Antibiotic Resistance - Major Concern

Resistance Mechanisms

Treatment by Resistance Profile

Risk Factors for Acquisition

• Hospitalization / LTCF residence
• Mechanical ventilation (VAP)
• Indwelling urinary catheter (complicated UTI)
• Diabetes mellitus
• Chronic alcoholism
• COPD / structural lung disease
• Immunosuppression (neutropenia, transplant, steroids)
• Broad-spectrum antibiotic use (selects for MDR strains)
• Asian origin / travel (for hvKp and NDR strains)

Epidemiology (2025 Update)

Summary Table