Lactose positive colonies of Citrobacter freundii on MacConkey agar. Cultivation 24 hours, aerobic atmosphere, 37°C. Approximately 20% of clinical isolates are lactose negative.
These bacteria can be found almost everywhere in soil, water, wastewater, etc. It can also be found in the human intestine. They are rarely the source of illnesses,
except for infections of the urinary tract and infant meningitis and sepsis.
C. freundii strains have inducible ampC genes encoding resistance to ampicillin and first-generation cephalosporins. In addition, isolates of Citrobacter may be resistant to multiple other antibiotics as a result of plasmid-encoded resistance genes.