genus of rod-shaped bacteria of the family Enterobacteriaceae. Klebsiella organisms are categorized microbiologically as gram-negative, facultative anaerobic, nonmotile bacteria. Klebsiella pneumoniae, also called Friedländer's bacillus, can infect the human respiratory tract and cause pneumonia, although the disease is usually seen only in patients with underlying medical problems such as alcoholism or chronic pulmonary disease. Traditionally the bacteria K. pneumoniae, K. ozaenae, and K. rhinoscleromatis were recognized as separate species, but DNA studies indicate that all three should be classified as subspecies of K. pneumoniae. For medical purposes the species distinctions are still observed, however. Other Klebsiella species include K. oxytoca and K. planticola, which, along with K. pneumoniae, can cause human urinary tract and wound infections.