Klebsiella pneumoniae is a Gram-negative, encapsulated, facultatively anaerobic rod-shaped bacterium and an opportunistic human pathogen.
This species belongs to the Enterobacteriaceae family and is commonly found in soil, water and the gastrointestinal tract of humans and animals. Its prominent polysaccharide capsule gives colonies a mucoid appearance and helps the organism evade phagocytosis and complement-mediated killing. K. pneumoniae can cause severe community- and hospital-acquired infections, including pneumonia, urinary tract infections, bloodstream infections and liver abscesses. Classical strains are associated with aspiration pneumonia in individuals with alcoholism or underlying pulmonary disease; infection often results in necrotising consolidation with thick, bloody sputum. Hypervirulent strains possessing rmpA and aerobactin genes have emerged, causing invasive liver abscesses, meningitis and endophthalmitis in otherwise healthy people.
Antimicrobial resistance is a major concern. Many clinical isolates produce extended-spectrum beta-lactamases (ESBLs) that inactivate third-generation cephalosporins. Carbapenemase-producing K. pneumoniae, such as those carrying the KPC and NDM genes, are resistant to most beta-lactam antibiotics and pose a threat in healthcare settings. Diagnosis relies on culture and identification from clinical specimens; colonies are large, mucoid and lactose-fermenting on MacConkey agar. Molecular tests detect resistance genes and virulence factors. Management includes appropriate antibiotic therapy guided by susceptibility testing, source control and infection prevention measures. Vaccines targeting capsular antigens are under development.
Pathogenesis and Clinical Examples
Hospital-acquired pneumonia caused by K. pneumoniae often occurs in ventilated patients or those with compromised immune systems. The organism can colonise respiratory equipment and spread via healthcare workers. Infections can progress rapidly, leading to sepsis and shock. Hypervirulent strains endemic in East Asia cause liver abscesses that may metastasise to the eye or central nervous system. Urinary tract infections often arise in catheterised patients. The bacterium is also implicated in neonatal sepsis and meningitis. A positive string test, in which a loop forms a viscous string when lifted from a colony, suggests a hypermucoviscous phenotype.
Klebsiella pneumoniae exemplifies how a commensal bacterium can become a dangerous pathogen when host defenses are impaired. Vigilant infection control and the development of new antibiotics are essential to counter the rise of multidrug-resistant strains.
Related Terms: Enterobacteriaceae, Gram-negative, Capsule, ESBL, Pneumonia