Skip to content
MicrobioHub
Back to database
Enterobacterales

Citrobacter koseri

Motile Gram-negative bacilli

Identification & Growth

Identification

  • Indole-positive, ornithine-positive, adonitol-positive; H₂S-negative
  • Distinguished from C. freundii by indole and the absence of H₂S

Growth Conditions

  • MacConkey, late lactose; 35–37 °C, 18–24 h

Clinical Significance

  • Neonatal meningitis and brain abscess; urinary tract infection in adults

Intrinsic Resistance

  • Intrinsic resistance to ampicillin and ticarcillin (class A chromosomal beta-lactamase)

Bench Alerts

  • Unlike C. freundii it is not a classical inducible AmpC producer — do not apply the AmpC group rule automatically
  • Isolation from neonatal CSF requires imaging because of the abscess risk

Clinical Notes

Clinical presentations

  • Causes severe neonatal meningitis, frequently complicated by brain abscess and ventriculitis.
  • Also causes urinary tract infections and bacteremia in hospitalized adult patients.
  • Can be associated with intra-abdominal and wound infections.

Specimens and collection

  • Cerebrospinal fluid should be urgently collected in a febrile neonate with neurologic signs before antibiotic therapy.
  • Neuroimaging (transfontanellar ultrasound or MRI) should accompany the investigation of neonatal meningitis by this organism.
  • Blood and urine cultures are indicated according to the suspected clinical site in adults.

Epidemiology and at-risk populations

  • Neonates, especially preterm infants, are the classic at-risk population for severe meningitis and brain abscess formation.
  • Found in the environment and human gastrointestinal tract as an occasional colonizer.
  • Neonatal unit outbreaks have been associated with cross-transmission via fomites and healthcare worker hands.

Resistance and therapeutic implications

  • Possesses a low-level inducible chromosomal AmpC beta-lactamase, with potential for hyperproduction under beta-lactam selective pressure.
  • Third-generation cephalosporins should be used cautiously in prolonged therapy due to the risk of emerging resistance.
  • Cefepime or carbapenems are preferable in severe, prolonged infections, such as complicated meningitis.

Bench and reporting notes

  • Differentiation from other Citrobacter species by biochemical profile (positive indole production) aids presumptive identification.
  • Isolation in a neonate's CSF should be communicated urgently to the care team given prognostic severity.
  • Flag the risk of emerging cephalosporin resistance during prolonged treatment in follow-up reports.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Citrobacter koseri in the lab?

Citrobacter koseri is identified through indole-positive, ornithine-positive, adonitol-positive; h₂s-negative, distinguished from c. freundii by indole and the absence of h₂s.

What are the intrinsic resistances of Citrobacter koseri?

This organism is naturally resistant to intrinsic resistance to ampicillin and ticarcillin (class a chromosomal beta-lactamase). These drugs should not be reported as susceptible.

Where is Citrobacter koseri commonly found?

It is typically associated with neonatal meningitis and brain abscess; urinary tract infection in adults.

Antibiogram InterpreterCheck BrCAST 2026 breakpointsRugai InterpreterBiochemical identification tools

Want the free Bench Kit?

Sign up to receive the PDF kit: agars, Gram, Rugai, dilutions and the antibiogram checklist.