Klebsiella are non-motile, rod-shaped, aerobic bacteria that possess a prominent polysaccharide capsule. Some of these bacteria produce an extracellular toxic complex that has been shown to be "lethal for and produce extensive lung pathology in mice". It is composed of 63% capsular polysaccharide, 30% lipopolysaccharide, and 7% protein; when introduced to experimental animals in sublethal doses, the animals built up immunization due to antibody production. Klebsiella pneumoniae is one of the most common Gram-negative bacteria seen by physicians worldwide. Pneumonias that are caused by Klebsiella pneumoniae are difficult to control; mortality rates have even been reported as up to 50% after antibiotic treatment. Nitrogen metabolism in Klebsiella aerogenes has also been studied throroughly.