Citrobacter freundii are 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 non-motile. It can be found in soil, water, sewage, food and the intestinal tracts of animals and humans. It is classified under the family of Enterobacteriaceae. The Citrobacter genus was discovered in 1932 by Werkman and Gillen. Cultures of C. freundii were isolated and identified in the same year from soil extracts.
As an opportunistic pathogen, C. freundii is responsible for a number of significant opportunistic infections. It is known to be the cause of a number of nosocomial infections of the respiratory tract, urinary tract, blood and many other normally sterile sites in patients. C. freundii represents about 29% of all opportunistic infections.
Surprisingly, this infectious microbe in humans plays a positive role in the environment. C. freundii is responsible for reducing nitrate to nitrite in the environment. This conversion is an important and crucial stage in the nitrogen cycle. The bacteria also helps in recycling nitrogen.
Citrobacter freundii has also been investigated for biodegradation of tannic acid used in tanneries.
For metabolism, C. freundii has an ability to grow on glycerol as the sole carbon and energy source. Within its cell, a bacterial microcompartment can be found, which is capable of processing propanediol.
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