Klebsiella pneumoniae is a Gram-negative, non-motile, enclosed, lactose fermenting, facultative anaerobic and rod-shaped bacterium residing in the common flora of the skin, mouth and intestines. Clinically it is a major member of the Klebsiella genus of Enterobacteriaceae and closely related to K. oxytoca. It commonly occurs in the soil and roughly 30% of strains can fix nitrogen in anaerobic condition, reason why its nitrogen fixation system has been the subject of much study.

Members of the Klebsiella genus generally express two types of antigens on their cell surface, O antigen, which is a lipopolysaccharide with 9 varities, and K antigen, which is a capsular polysaccharide with over 80 varities, both equally contributing to pathogenicity and forming the grounds for subtyping.

Danish scientist Hans Christian Gram (1853-1938) came up with the technique known today as Gram staining in 1884 to differentiate K. pneumoniae and Streptococcus penumoniae. Klebsiella derives from the name of German bacteriologist Edwin Klebs (1834-1913).

The highly resistant K. pneumoniae have been destroyed in vivo through intraperitoneal, intravenous and intranasal administration of phages during laboratory tests.

Clinically, it might be the cause of Klebsiella pneumonia. Typically, Klebsiella infections have the tendency to occur in people with a poor immune system caused by inadequate diet, such as alcoholics or diabetics. A great number of these infections are acquired by people hospitalized for some reason, a nosocomial infection. The most frequent infection that occurs outside the hospital by Klebsiella bactaria is pneumonia.

New antibiotic resistant strains of K. pneumoniae are constantly surfacing and it is more and more found as a nosocomial infection.

Klebsiella is on the second place after E. coli for urinary tract infections with older people and in the same time it is an opportunistic pathogen for patients suffering from chronic pulmonary disease, enteric pathogenicity, rhinoscleroma and nasal mucosa atrophy. Feces are the main source of patient infection, next to contact with infected instruments.