Citrobacter spp.


  1. Gram -ve bacilli (non spore forming).
  2. Aerobic respiration, facultative anaerobes.
  3. Motile via peritrichous flagella.
  4. Indole +ve.
  5. Citrobacter freundii has been investigated for biodegradation of tannic acid (used in tannerys).
  6. Citrobacter freundii can be isolated from soil, water, sewage, and food.
  7. Citrobacter freundii is often found in clinical specimens as an opportunistic or secondary pathogen.
Biochemical Properties:
Members of the genus Citrobacter are Gram-negative, nonsporeforming rods belonging to the family Enterobacteriaceae and, as the name suggests, usually utilize citrate as a sole carbon source. These facultative anaerobes typically are motile by means of peritrichous flagella. They ferment glucose and other carbohydrates with the production of acid and gas. They are oxidase negative, catalase and methyl red positive, Voges-Proskauer negative, and do not decarboxylate lysine. They are differentiated by their ability to convert tryptophan to indole, ferment lactose, and utilize malonate. Of the dozen species, C. freundii, C. diversus, and C. amalonaticus are linked to human disease. Taxonomically, the genus Citrobacter is most closely related to Salmonella and Escherichia.



  1. The urinary tract is the most frequent site from which Citrobacter is cultured, often in association with an indwelling catheter. These bacteria may also be cultured from the respiratory tract, a finding that more often represents colonization than symptomatic infection.
  2. Citrobacter strains are also involved in intra-abdominal infections and can cause soft tissue infections and osteomyelitis.
  3. Invasive procedures may play a role in Citrobacter bacteremia.
  4. C. diversus has caused frequent nosocomial outbreaks of neonatal meningitis. In several instances the outbreaks have been accompanied by high rates of intestinal colonization in infants by the organism and by carriage of the bacteria on the hands of health care workers.
  5. C. freundii strains, like strains of Enterobacter and Serratia, have inducible ampC genes encoding resistance to ampicillin and first-generation cephalosporins that can be produced constitutively at high levels following mutations. In addition, like members of these other genera, isolates of Citrobacter may be resistant to multiple other antibiotics as a result of plasmid-encoded resistance genes. See ESBLs.
Just to add, the patient had multiple hepato-splenic abscesses and we identified via VITEK2, C. koseri. Citrobacter infection is commonly reported in neonates and immunocompromised patients. Citrobacter koseri (diversus) is an important cause of neonatal meningitis and brain abscess formation. It adults, however, Citrobacter infection with central nervous system involvement is rare, and is usually associated with an underlying disorder. C. koseri is regarded as an opportunistic pathogen. In immunocompromised hosts, C. koseri not only colonized patients, but also caused bacteremia, urinary tract infection, enteritis, and peritonitis. The number of C. koseri infections worldwide has increased significantly, and nosocomial transmission appears to have occured, despite standard infection control measures. A Japanese group introduced the following precautions:
  1. Barrier precautions against infected patients & colonized patients, using disposable gloves and drapes.
  2. Mandatory hand washing before and after any manipulation involved in the nursing care.
  3. Hand hygiene not only for medical workers but also for patients.
  4. Water taps were converted to the hands-free types, and all door knobs and bars for drip injection were sterilized using 70% alcohol twice daily.
  5. Restriction of the prophylactic use of antibiotics for neutropaenic patients.