A genus of gram-negative, nonmotile bacteria whose members ferment lactose, do not decarboxylate ornithine or form hydrogen sulfide, and often produce large mucoid colonies and gas from glucose. Characteristic large mucoid colonies are due to production of a large amount of capsular material. Some isolates, particularly those from urinary tract infections, produce a slow-acting urease enzyme. Species of Klebsiella are commonly found in soil and water, on plants, and in animals and humans. Harmless strains of Klebsiella are beneficial because they fix nitrogen in soil. Pathogenic species include K. pneumoniae, K. rhinoscleromatis, and  K. ozaenae, also known as K. pneumoniae subspecies pneumoniae, rhinoscleromatis, and ozaenae. Species and strains can be differentiated by biochemical reactions and typed by bacteriocin susceptibility and serologically by antigen.