Klebsiella is a genus of Gram-negative, oxidase-negative, rod-shaped bacteria with a prominent polysaccharide-based capsule. Klebsiella species are found everywhere in nature. This is thought to be due to distinct sublineages developing specific niche adaptations, with associated biochemical adaptations which make them better suited to a particular environment. They can be found in water, soil, plants, insects and other animals including humans. 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 members of the genus Klebsiella are a part of the human and animal's normal flora in the nose, mouth and intestines. 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. The cells are rods in shape and generally measures 0.3 to 1.5 µm wide by 0.5 to 5.0 µm long. They can be found singly, in pairs, in chains or linked end to end. Klebsiella can grow on ordinary lab medium and do not have special growth requirements, like the other members of Enterobacteriaceae. The species are aerobic but facultatively anaerobic. Their ideal growth temperature is 35° to 37 °C, while their ideal pH level is about 7.2. List of species K. aerogenes, previously known as Enterobacter aerogenes and Bacterium aerogenes K. granulomatis K. oxytoca K. michiganensis K. pneumoniae (type-species) K. p. subsp. ozaenae K. p. subsp. pneumoniae K. p. subsp. rhinoscleromatis K. quasipneumoniae K. q. subsp. quasipneumoniae K. q. subsp. similipneumoniae K. grimontii K. variicola Features Klebsiella bacteria tend to be rounder and thicker than other members of the family Enterobacteriaceae. They typically occur as straight rods with rounded or slightly pointed ends. They can be found singly, in pairs, or in short chains. Diplobacillary forms are commonly found in vivo. They have no specific growth requirements and grow well on standard laboratory media, but grow best between 35 and 37 °C and at pH 7.2. The species are facultative anaerobes, and most strains can survive with citrate and glucose as their sole carbon sources and ammonia as their sole nitrogen source. Members of the genus produce a prominent capsule, or slime layer, which can be used for serologic identification, but molecular serotyping may replace this method. Members of the genus Klebsiella typically express two types of antigens on their cell surfaces. The first, O antigen,