Klebsiella Pneumoniae

Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose fermenting, facultative anaerobic, rod shaped bacterium found in the normal flora of the mouth, skin, and intestines.

It is clinically the most important member of the Klebsiella genus of Enterobacteriaceae.

Seven species of the Klebsiella genus, with demonstrated similarities in DNA homology are known. These are (1) Klebsiella pneumoniae, (2) Klebsiella ozaenae, (3) Klebsiella terrigena, (4) Klebsiella rhinoscleromatis, (5) Klebsiella oxytoca, (6) Klebsiella planticola, and (7) Klebsiella ornithinolytica. Of these, K oxytoca and K rhinoscleromatis have also been demonstrated in human clinical specimens.

In recent years, klebsiellae have become important pathogens in nosocomial infections.

It is closely related to K. oxytoca from which it is distinguished by being indole-negative and by its ability to grow on both melezitose and 3-hydroxybutyrate. It naturally occurs in the soil, and about 30% of strains can fix nitrogen in anaerobic conditions. As a free-living diazotroph, its nitrogen fixation system has been much studied.

Members of the Klebsiella genus typically express 2 types of antigens on their cell surface. The first, O antigen, is a component of the lipopolysaccharide (LPS), of which 9 varieties exist. The second is K antigen, a capsular polysaccharide with more than 80 varieties. Both contribute to pathogenicity and form the basis for serogrouping.