This scanning electron micrograph (SEM) reveals some of the ultrastructural morphologic features of Klebsiella pneumoniae. Courtesy of CDC/Janice Carr.
This scanning electron micrograph (SEM) reveals some of the ultrastructural morphologic features of Klebsiella pneumoniae. Courtesy of CDC/Janice Carr.
The genus Klebsiella belongs to the tribe Klebsiellae, a member of the family Enterobacteriaceae. The organisms are named after Edwin Klebs, a 19th century German microbiologist. Klebsiellae are nonmotile, rod-shaped, gram-negative bacteria with a prominent polysaccharide capsule. This capsule encases the entire cell surface, accounts for the large appearance of the organism on gram stain, and provides resistance against many host defense mechanisms.
Members of the Klebsiella genus typically express 2 types of antigens on their cell surface. The first is a lipopolysaccharide (O antigen); the other is a capsular polysaccharide (K antigen). Both of these antigens contribute to pathogenicity. About 77 K antigens and 9 O antigens exist. The structural variability of these antigens forms the basis for classification into various serotypes. The virulence of all serotypes appears to be similar.
The genus was originally divided into 3 main species based on biochemical reactions. Today, 7 species with demonstrated similarities in DNA homology are known. These are (1) Klebsiella pneumoniae, (2) Klebsiella ozaenae, (3) Klebsiella rhinoscleromatis, (4) Klebsiella oxytoca, (5) Klebsiella planticola, (6) Klebsiella terrigena, and (7) Klebsiella ornithinolytica. K pneumoniae is the most medically important species of the group. K oxytoca and K rhinoscleromatis have also been demonstrated in human clinical specimens. In recent years, klebsiellae have become important pathogens in nosocomial infections.1
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