Citrobacter koseri, formerly known as Citrobacter diversus, is a Gram-negative non-spore forming, rod-shaped bacterium. It is a facultative anaerobe capable of aerobic respiration. It is motile via peritrichous flagella. It is a member of the family of Enterobacteriaceae. The members of this family are part of the normal flora and commonly found in the digestive tracts of humans and animals.^[1] C. koseri may act as an opportunistic pathogen in individuals who are immunocompromised.

It rarely is community-acquired and mainly occurs as hospital-acquired infections. Infections caused by C. koseri can lead to various symptoms, including fever, chills, diarrhea, and abdominal pain. In severe cases, the bacterium can cause sepsis, meningitis, or brain abscesses. Brain abscesses have a high rate of mortality and complications, particularly in neonates. The transmission of C. koseri could be vertical from mother to fetus, and other sources can be horizontal by asymptomatic nursery staff.

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