Klebsiella are encapsulated gram-negative, lactose-fermenting bacilli. They form pink colonies on MacConkey agar due to lactose fermentation. The main virulence factor is a polysaccharide capsule. Klebsiella pneumoniae (the most important pathogenic species) is commonly associated with lobar aspiration pneumonia in patients with alcohol-use disorder. It also causes nosocomial infections, such as hospital-acquired pneumonia and urinary tract infections, as well as tissue-destructing abscesses (liver, lung, spleen). Treatment is with cephalosporins for sensitive strains. However, many Klebsiella species have plasmids that confer multidrug resistance.

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Gram negative bacteria classification flowchart

Gram-negative bacteria:
Most bacteria can be classified according to a lab procedure called Gram staining.
Bacteria with cell walls that have a thin layer of peptidoglycan do not retain the crystal violet stain utilized in Gram staining. These bacteria do, however, retain the safranin counterstain and thus appear as pinkish-red on the stain, making them gram negative. These bacteria can be further classified according to morphology (diplococci, curved rods, bacilli, and coccobacilli) and their ability to grow in the presence of oxygen (aerobic versus anaerobic). The bacteria can be more narrowly identified by growing them on specific media (triple sugar iron (TSI) agar) where their enzymes can be identified (urease, oxidase) and their ability to ferment lactose can be tested.
* Stains poorly on Gram stain
** Pleomorphic rod/coccobacillus
*** Require special transport media

Image by Lecturio.

General Characteristics

General characteristics

  • Gram-negative bacilli
  • Facultative anaerobes
  • Non-motile
  • Encapsulated (polysaccharide capsule)
  • Single, in pairs, or short chains
  • Form large, mucoid colonies
  • Major pathogenic species: Klebsiella pneumoniae


  • Lactose fermenters:
    • Ferment lactose to lactic acid
    • Pink colonies on MacConkey agar
  • Urease positive: hydrolysis of urea into ammonia and carbon dioxide
  • Oxidase negative
  • Can use citrate and glucose as sole carbon sources
  • Can use ammonia as a sole nitrogen source


  • Water/soil
  • Plants, animals, and insects
  • Normal flora of human nose, mouth, and gastrointestinal tract

Virulence factors

  • Polysaccharide capsule:
    • Evades phagocytosis
    • 77 different capsule serotypes
    • K1 and K2 types are the most virulent.
  • Hypermucoviscosity phenotype:
    • Production of mucoviscous exopolysaccharide web
    • Associated with liver abscesses and other destructive tissue abscesses
    • String test: > 5-mm viscous string when the colony on agar plate is stretched using a bacteriologic loop
  • Lipopolysaccharide:
    • Lipopolysaccharide type O side chain
    • Protects bacteria from complement-mediated membrane damage
  • Siderophores:
    • Iron chelators
    • Enhance iron uptake, which is vital for Klebsiella growth
  • Pili (fimbriae):
    • Mediate adherence to epithelial and mucosal surfaces
    • Formation of treatment-resistant biofilms

Clinical Relevance

Epidemiology and transmission

  • Carrier rates:
    • 5%–38% intestinal colonization
    • 1%–6% nasopharyngeal 
    • Higher carrier rates:
      • Individuals with alcohol-use disorder (nasopharyngeal)
      • Chinese ethnicity (intestinal)
      • Hospitalized patients
  • Transmission:
    • Nosocomial infection most common
    • Community-acquired less frequent
    • Person-to-person
    • Associated with aspiration (pneumonia)
    • Contamination from the environment (rare)
  • Risk factors:
    • Alcohol-use disorder (community-acquired pneumonia)
    • Impaired host defenses:
      • Hospitalization, especially if catheterized or intubated
      • Diabetes
      • Severe chronic obstructive pulmonary disease

Infectious manifestations


  • 7.6% of hospital-acquired pneumonias (intubated and non-intubated patients)
  • In community-acquired cases, predilection for posterior segments of right upper lobe
  • Currant-jelly” sputum
  • Fever, cough, dyspnea, pleuritic chest pain
  • Complications:
    • Lung abscess (may mimic tuberculosis on chest X-ray)
    • Empyema

Urinary tract infections (UTIs):

  • 3rd leading cause of UTIs
  • Can cause lower (cystitis) and upper (pyelonephritis) UTIs
  • May be associated with ammonium magnesium phosphate (struvite) renal stones
  • In hospitalized patients with urinary catheters

Catheter-related bacteremia:

  • Associated with central venous catheters
  • Relatively infrequent cause (approximately 5%)

Less frequent infections:

  • Intra-abdominal:
    • Liver abscess
    • Spleen abscess
    • Spontaneous bacterial peritonitis
  • Meningitis: usually associated with neurosurgical procedures
  • Endocarditis
  • Endophthalmitis
  • Skin and soft-tissue infections
Radiological (A) and pathological (B) representation of Klebsiella pneumonia

Radiological (A) and pathological (B) representation of Klebsiella pneumoniae: prominent voluminous hyperdense, symmetric bilateral lung consolidation, and the massive infiltration of neutrophils in the interstitium and intra-alveolar spaces

Image: “Fulminant Klebsiella pneumoniae pneumonia in an immunocompetent patient without alcohol-use disorder” by Benyashvili Z, Djirbe A, Assy N. License: CC BY 3.0


To help remember the clinical manifestations of Klebsiella, remember “the 5 As of KlebsiellA:”

  • Aspiration pneumonia
  • Alcohol-use disorder
  • Abscesses in lungs and liver
  • Di-A-betes
  • “Curr-A-nt jelly” sputum

Antibiotic resistance

  • Some strains produce extended-spectrum beta-lactamases (ESBL).
  • Resistance to most beta-lactam antibiotics: cephalosporins, penicillins, monobactams, carbapenems
  • Some strains are also cross-resistant to other antibiotics (multidrug resistance).
  • Major risk factor: prior antibiotic exposure
  • Mediated by plasmids that encode beta-lactamases and other modifying enzymes
  • Plasmids exchanged via pili
Plasmid exchange in Klebsiella

Plasmid exchange in Klebsiella
A pilus allows for the exchange of replicated plasmid between 2 cells. As soon as the pilus is formed, the plasmid is replicated and then transferred through the pilus from the donor into the recipient cell.

Image by Lecturio.


  • Gram stain:
    • Sputum: sensitivity about 50%
  • Cultures:
    • Blood
    • Urine
    • Body fluids
    • Abscess aspirates


  1. Wen-Liang Y., Yin-Ching C. (2019). Clinical features, diagnosis, and treatment of Klebsiella pneumoniae infection. Retrieved 1 January 2021, from https://www.uptodate.com/contents/clinical-features-diagnosis-and-treatment-of-klebsiella-pneumoniae-infection
  2. Wen-Liang Y., Yin-Ching C. (2019). Microbiology and pathogenesis of Klebsiella pneumoniae infection. Retrieved 1 January 2021, from https://www.uptodate.com/contents/microbiology-and-pathogenesis-of-klebsiella-pneumoniae-infection

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