Citrobacter freundii complex

• C. braakii
• C. freundii
• C. gillenii
• C. murliniae

• C. sedlakii

• C. werkmanii
• C. youngae

Gram Stain

• Gram negative bacilli - fermenter

Clinical Significance

These organisms are found in the gastrointestinal tract of humans and animals. Their presence in water, soil and food sources probably reflects fecal contamination.

Organisms in this group are important opportunistic pathogens associated with nosocomial infections including bacteremia, peritonitis, meningitis, and urinary tract, respiratory tract, bone/joint and skin/soft tissue infections. 

They can also cause endocarditis.

Septicemia with these organisms tends to be polymicrobial and associated with high morbidity and mortality.

Usual Susceptibility Pattern

These organisms produce an inducible cephalosporinase (AmpC) rendering them resistant to ampicillin  and 1st /2nd  generation cephalosporins.  

Therapy with 3rd generation cephalosporins is not recommended as clinical failures can occur despite in vitro susceptibility.

Note: Beta-lactamase inhibitor combinations are not recommended as the cephalosporinase is not inhibited by beta-lactamase inhibitor combination drugs.

They are typically susceptible to ciprofloxacin, aminoglycosides, nitrofurantoin, fosfomycin, and carbapenems (meropenem has better activity than imipenem).

Susceptibility to TMP/SMX is variable.