(Braak 1928) Werkman and Gillen 1932
Citrobacter freundii is a species of facultative anaerobic gram-negative bacteria of the family Enterobacteriaceae. The bacteria have a long rod shape with a typical length of 1–5 μm. Most C. freundii cells generally have several flagella used for locomotion, but some do not and are non-motile. C. freundii is a soil organism, but can also be found in water, sewage, food and in the intestinal tracts of animals and humans. The genus Citrobacter was discovered in 1932 by Werkman and Gillen. Cultures of C. freundii were isolated and identified in the same year from soil extracts.
C. freundii is a common component of the gut microbiome of healthy humans. While most strains are beneficial, there are significant phenotypic variations among strains, even those that share >99% of their genome. Some rare strains of C. freundii have been associated with opportunistic nosocomial infections of the respiratory tract, urinary tract, blood, and many other normally sterile sites in immunocompromised patients.
C. freundii is also commonly found to be a member of the soil microbiome. This microbe plays an important role in the nitrogen cycle 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.
Some members of the species are nitrogen-fixing and have been found in tissues of living sassafras trees.
C. freundii has the ability to grow on glycerol, and use it as its sole source of carbon and energy. The organism contains a bacterial microcompartment which is capable of processing propanediol. C. freundii creates a positive MR and negative VP test along with a positive Catalase and negative Oxidase test. C. freundii cannot hydrolyze starch, lipids, or gelatin. 
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