Klebsiella Pneumoniae: Classification, Structure, Virulence factor, Pathogenesis and Laboratory Diagnosis
History of Discovery
Rod-shaped bacteria with massive capsule – causative agents of severe pneumonia cases – were primarily described by the German bacteriologist E. Klebs in 1875. They were further isolated in pure culture by the German pathologist C. Friedlander in 1882, and the pathogen was named Klebsiella pneumoniae.
Classification of Klebsiellae
- These bacteria pertain to the family Enterobacteriaceae, genus Klebsiella; basic pathogenic species of the genus are K. рneumoniae and K. оxytoca.
- In 2001 many former klebsiella representatives were placed into separate genus Raoultella. It includes species R. terrigena, R. planticola, R. ornithinolytica and others that inhabit soil, water, or plants.
- According to the modern phylogenetic analysis, Klebsiella genus incorporated another human pathogen previously known as Calymmatobacterium granulomatis. Currently this bacterium is called as Klebsiella granulomatis. It causes endemic sexually transmitted disease “granuloma inguinale” or donovanosis.
- Type species of the genus Klebsiella with evident pathogenic potential is K. pneumoniae. Its subsequent genetic analysis revealed 3 distinct subspecies – K. pneumoniae, K. ozaеnae, and K. rhinoscleromatis.
Structure and Properties of K. pneumoniae
- The bacteria represent thick short gram-negative rods arranged as single cells, pairs (diplobacteria) or chains. Microbial cells possess large polysaccharide capsule. They lack spores or flagella, but carry multiple fimbrias (pili) with prominent adhesive capacity to epithelial cells.
- Klebsiella easily grows on ordinary media demonstrating mucous dome-shaped colonies. Optimal growth temperature is 35-37oС, рН 7.2.
- As all enterobacteria, they are facultative anaerobes.Klebsiellae have somatic O-antigen (8 serovars) and capsular К-antigen (above 80 serovars). Serovar typing rests on variations of K-Ag.
Differentiation of klebsiellae is based on a number of biochemical tests.
- Multiple fimbrial adhesins account for binding to epithelial cells.
- Thick capsule protects bacteria against phagocytosis.
- Endotoxin (LPS of bacterial cell wall) stimulates inflammatory reactions.
- Siderophore proteins (aerobactin, salmochelin and others) deliver iron for successful microbial growth.
- Minor part of bacterial strains is capable of producing enterotoxin or hemolysin.
- Pathogenic klebsiella are hallmarked with intensive production of antibiotic-degrading enzymes – extended-spectrum β-lactamases (ESBLs) and carbapenemases. This leads to prominent multidrug resistance of bacterial cells. Antimicrobial resistance is encoded by virulence plasmids.
- Klebsiellae are easily inactivated by temperature 80-100°С. Similarly, they are sensitive to most of conventional disinfectants (e.g., chloramine, phenol and others).
Pathogenesis and Clinical Findings in Klebsielloses
- K. рneumoniae and K. оxytoca are common agents of opportunistic infections. Overall, they cause about 5-8% of total cases of nosocomial infections in hospital health care settings.
- Klebsiella easily colonize mucosal tissues. Humans carry these pathogens in nasopharyngeal cavity and intestinal tract.
- K. рneumoniae subspecies is strongly associated with severe pneumonia that develops in patients with suppressed cell-mediated innate immunity, e.g. phagocytosis deficiency. Mortality rate in these cases may exceed 50%. A key role in survival of bacterial cell under the immune pressure pertains to microbial capsule.
- The bacteria also cause lung abscesses, urogenital disorders, soft tissue infections, infections of artificial appliances (catheters, drains, etc.)
- More seldom they cause enteritis, arthritis, meningitis, or generalized infection (sepsis).
- Many hospital isolates of K. рneumoniae demonstrate multiple drug resistance due to the production of extended-spectrum β-lactamases (ESBLs) and carbapenemases.
- Evolution of these bacteria resulted in emergence of extremely drug-resistant K. pneumoniae (XDR-KP) that provokes the most severe outbreaks of hospital-acquired infections resistant to antimicrobial therapy.
- Quite recently the global spread of another highly dangerous pathotype of K. pneumoniae was registered. It causes community-acquired systemic infections affecting immunocompetent individuals.
- This variant was termed as hypervirulent K. pneumoniae (hvKP). It is characterized by enhanced capsule production (hypermucoviscous phenotype) and overexpression of iron-acquisition siderophores, primarily aerobactin.
- Hypervirulent K. pneumoniae leads to potentially lethal infections with strong tendency to methastatic spread in immunocompetent persons. Patients demonstrate pyogenic liver abscesses, endophthalmitis, or meningitis that may progress towards generalized systemic disease (sepsis).
- Current findings of hvKP with multdrug resistance pose a serious threat for public health state.
- Subspecies K. ozaеnae causes rare chronic infection of upper respiratory tract known as ozaena. This disease is a special clinical form of chronic atrophic rhinitis. Propagating bacteria stimulate fetid nasal discharge and formation of thick foul crusts in nasal cavities. The pharynx, larynx and large bronchi can be affected as well.
- Subspecies K. rhinoscleromatis causes another rare disease of upper respiratory tract, namely rhinoscleroma. It is a chronic granulomatous infection, predominantly affecting nasal cavity. Similarly, it afflicts oropharingeal area, larynx, and bronchial tree.
- Chronic infection results in development of intranasal nodules (polips) with subsequent destruction of nasal cartilage and nose deformity. Late course of the disease demonstrates progressive sclerosis and fibrosis in nasal cavity and upper airways.
The cases of ozaena and rhinoscleroma were sometimes registered on the territory of Belarus.
- K. oxytoca causes hospital-acquired (nosocomial) opportunistic infections of lower respiratory tract, urinary tract, purulent wound infections. Hospital strains of K. oxytoca possess multiple antimicrobial resistance.
- Immunity in klebsielloses is of moderate grade and unstable. Phagocytes play decisive role in the control of infection.
Laboratory Diagnosis of Klebsielloses
- Various specimens (e.g., sputum, bronchial aspirate, wound discharge, urine, etc.) are used for laboratory examination.
- Microscopy is applied as a preliminary test. Rod-shaped bacteria surrounded by transparent large capsule are visualized by Gin’s staining method. Immunofluorescence microscopy can also be used.
- Culture isolation is performed by specimen inoculation into lactose agar with penicillin and bromine thymol blue indicator. Klebsiellae produce large mucous yellow colonies due to lactose fermentation.
- Differentiation of klebsiella species is elaborated according to the results of multiple biochemical tests.
- Type-specific identification of bacteria (for instance, detection of hypervirulent strains) includes serological agglutination tests with type-specific K-antisera. Most of hypervirulent isolates of K. pneumoniae pertain to K1 or K2 serotypes.
- Molecular genetic methods are actively used both for detection of multidrug resistant or hypervirulent strains of klebsiellae. Antimicrobial resistance is also determined by phenotypic susceptibility testing (diffusion and dilution tests).
Treatment and Prophylaxis of Klebsielloses
- Treatment of community-acquired K. pneumoniae infection includes third generation cephalosporins, fluoroquinolones, and aminoglycosides.
- Hospital ESBL-producing strains of K. pneumoniae are treated with carbapenems.
- Infections caused by carbapenemase-producing K. pneumoniae are remarkably difficult for treatment. Strongly limited number of antimicrobial agents (tigecycline or colistin) is efficient in these cases.
- Prophylaxis of infections associated with various klebsiella species is non-specific.