The Genus Citrobacter Background Information

 

Citrobacter is a distinct group of Gram-negative bacilli belonging to the family Enterobacteriaceae. The morphology of the bacterial cell consists of right bacilli, insulated or grouped in pairs and 1.0 micrometer in diameter and 2.0 to 6.0 micrometer in length. Citrobacter is facultatively anaerobic, motile and uses citrate as sole carbon source. In 1993, the genus Citrobacter was classified into 11 distinct species.  

bullet

. Citrobacter amalonaticus 

bullet

. Citrobacter braakii 

bullet

. Citrobacter farmeri 
. Citrobacter freundii 
. Citrobacter gillenii.

bullet

  Citrobacter koseri
. Citrobacter murliniae 
. Citrobacter rodentium 
. Citrobacter sedlakii

bullet

. Citrobacter werkmanii 

bullet

. Citrobacter youngae 

Citrobacter colonies bare a strong resemblance to E. coli colonies in morphology. They are generally smooth, slightly convex translucent or opaque colonies with regular contour. Colonies can also have a rough or mucous aspect. Diameters of each colony are between 2 mm to 4 mm after 24 hours of incubation at 35˚C or 37˚C.  

Species of Citrobacter share many general characteristics of their family Enterobacteriaceae such as they are all non-spore forming, all ferment glucose, all reduce nitrates to nitrites, all are oxidase negative and  most are motile with peritrichous flagella. They are differentiated from other members of their family and from E. coli by their ability to use citrate as a carbon source as well as their inability to produce acetylmethylcarbinol and lysine.

 Citrobacter is commonly found in soil, water, food and the intestinal tracts of animals and humans. Citrobacter is not considered to be an enteric pathogen because it is part of the normal gut flora. This group of bacteria is of small clinical interest.

 Importance of Isolating Citrobacter

bulletC. freundii  is suspected to cause diarrhea and possibly extraintestinal infections
bulletC. diversus  has been linked to a few cases of meningitis in newborns
bulletC. braakii  can cause sepsis
bulletCitrobacter species are increasingly being classified as immunocompromised human pathogens instead of harmless contaminants.
bulletIt is becoming common to find Citrobacter  in drinking and household water sources due to increases in animal agriculture to feed the growing population