Klebsiella is a genus of Gram-negative, oxidase-negative, rod-shaped bacteria with a prominent polysaccharide-based capsule.
Klebsiella is named after German-Swiss microbiologist Edwin Klebs (1834–1913). Carl Friedlander described Klebsiella bacillus which is why it was termed Friedlander bacillus for many years. The species of Klebsiella are all gram-negative and usually non-motile. They tend to be shorter and thicker when compared to others in the family Enterobacteriaceae.
Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose-fermenting, facultative anaerobic, rod-shaped bacterium. It appears as a mucoid lactose fermenter on MacConkey agar.
Although found in the normal flora of the mouth, skin, and intestines, it can cause destructive changes to human and animal lungs if aspirated, specifically to the alveoli, resulting in bloody, brownish or yellow colored jelly-like sputum. In the clinical setting, it is the most significant member of the genus Klebsiella of the Enterobacteriaceae. K. oxytoca and K. rhinoscleromatis have also been demonstrated in human clinical specimens. In recent years, Klebsiella species have become important pathogens in nosocomial infections.
Enterobacteriaceae is a large family of Gram-negative bacteria. It includes over 30 genera and more than 100 species. Its classification above the level of family is still a subject of debate, but one classification places it in the order Enterobacterales of the class Gammaproteobacteria in the phylum Pseudomonadota. In 2016, the description and members of this family were emended based on comparative genomic analyses by Adeolu et al.
Enterobacteriaceae includes, along with many harmless symbionts, many of the more familiar pathogens, such as Salmonella, Escherichia coli, Klebsiella, and Shigella. Other disease-causing bacteria in this family include Enterobacter and Citrobacter. Members of the Enterobacteriaceae can be trivially referred to as enterobacteria or "enteric bacteria", as several members live in the intestines of animals. In fact, the etymology of the family is enterobacterium with the suffix to designate a family (aceae)—not after the genus Enterobacter (which would be "Enterobacteraceae")—and the type genus is Escherichia.