Klebsiella is a genus of Gram negative, rod-shaped bacteria in the family Enterobacteriaceae. They are facultative anaerobes and are found in the intestinal and repiratory tracts of many vertebrates (including humans). They are often responsible for bladder, kidney, lung, and wound infections. One particular species, K. pneumoniae, can cause pneumonia in people with AIDS or otherwise-compromised immune systems.

The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.