Citrobacter freundii is a species of facultative anaerobic. Gram-negative bacilli of the Enterobacteriaceae family. The bacteria are long rod-shaped with a typical length of 1–5 μm. Most C. freundii cells are surrounded by several flagella used for locomotion, but a few are not mobile.It can be found in soil, water, sewage, food, and the intestinal tracts of animals and humans.. freundii is responsible for reducing nitrate to nitrite in the environment. This conversion is an important and crucial stage in the nitrogen cycle. These bacteria also help in recycling nitrogen. C. freundii has also been investigated for biodegradation of tannic acid used in tanneries.