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Enterobacterales

Citrobacter freundii

Gram-negative bacilli

Identification & Growth

Identification

  • Oxidase-negative, citrate-positive, H₂S-positive, indole-negative, lysine-negative
  • May be confused with Salmonella on TSI

Growth Conditions

  • MacConkey agar: late lactose fermentation; 35 °C, 24 h

Clinical Significance

  • UTI, wound infection, hospital-acquired bacteremia

Intrinsic Resistance

  • Ampicillin, amoxicillin-clavulanate, cefalotin, cefoxitin (chromosomal AmpC, EUCAST Expected Resistant Phenotypes v1.2)

Bench Alerts

  • Lysine-negative and ONPG-positive results separate it from Salmonella — confirm with serology on stool isolates
  • Inducible AmpC: the same caution applies as for Enterobacter

Clinical Notes

Clinical presentations

  • Causes urinary tract, bloodstream, and intra-abdominal infections, predominantly healthcare-associated.
  • Can cause neonatal meningitis and brain abscess, historically associated with Citrobacter koseri, though C. freundii is also implicated.
  • Frequently isolated from patients with catheters, invasive devices, or prolonged ICU stay.
  • May colonize the gastrointestinal tract before causing invasive infection in immunocompromised patients.

Specimens and collection

  • Midstream or catheter urine is the most common specimen for urinary infection.
  • Paired blood cultures should be drawn before starting empiric antibiotics when bacteremia is suspected.
  • Cerebrospinal fluid should be processed immediately when neonatal meningitis is suspected.
  • Wound or abscess specimens should be collected by aspiration, avoiding contaminated superficial swabs.

Epidemiology and at-risk populations

  • More common in elderly, hospitalized patients or those with structural urologic comorbidities.
  • Neonates are a specific at-risk population for severe meningitis with brain abscess formation.
  • Hospital outbreaks have been linked to contamination of solutions, equipment, and wet surfaces.

Resistance and therapeutic implications

  • Harbors inducible chromosomal AmpC, with risk of emergent resistance to broad-spectrum cephalosporins during therapy.
  • Isolates may acquire carbapenemases and other plasmid-mediated resistance mechanisms in the hospital setting.
  • Susceptibility testing should be interpreted considering the potential for AmpC derepression even with initially susceptible MICs.

Bench and reporting notes

  • Biochemical identification or mass spectrometry (MALDI-TOF) differentiates C. freundii from other genus species.
  • Reports of bacteremia with AmpC producers should flag the risk of therapeutic failure with third-generation cephalosporins.
  • Colonies on MacConkey agar are typically lactose-positive, requiring differentiation from other Enterobacterales.

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 freundii in the lab?

Citrobacter freundii is identified through oxidase-negative, citrate-positive, h₂s-positive, indole-negative, lysine-negative, may be confused with salmonella on tsi.

What are the intrinsic resistances of Citrobacter freundii?

This organism is naturally resistant to ampicillin, amoxicillin-clavulanate, cefalotin, cefoxitin (chromosomal ampc, eucast expected resistant phenotypes v1.2). These drugs should not be reported as susceptible.

Where is Citrobacter freundii commonly found?

It is typically associated with uti, wound infection, hospital-acquired bacteremia.

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