What is Citrobacter koseri?
Citrobacter koseri (formerly Citrobacter diversus) is a Gram-negative, motile, facultative anaerobic bacillus from the Enterobacteriaceae family that normally inhabits the human gastrointestinal tract but acts as an opportunistic pathogen primarily in neonates, elderly patients, and immunocompromised hosts. 1, 2
Microbiological Characteristics
- C. koseri is a non-spore-forming bacillus that utilizes citrate as its sole carbon source and constitutes part of the normal gastrointestinal flora in humans and animals 1
- It is classified within the Enterobacteriaceae family alongside other common pathogens such as E. coli, Klebsiella pneumoniae, and Klebsiella oxytoca 3
- The organism can be identified using 16S rRNA gene sequencing for definitive microbiological diagnosis 4
Clinical Significance and Infection Patterns
High-Risk Populations
- Neonates represent the highest-risk group, with C. koseri being one of the main etiological agents of neonatal meningitis and cerebral abscess 1
- In newborns, infection causes meningitis, cerebral abscesses, brain adhesions, encephalitis, and pneumocephalus, acquired through vertical maternal transmission or horizontal hospital settings 5
- Elderly individuals and immunocompromised patients (including those on long-term corticosteroid therapy) are particularly susceptible to severe infections 1
- Importantly, cases are increasingly reported in immunocompetent adults with no identifiable risk factors, challenging the traditional view of C. koseri as purely opportunistic 4, 6
Common Infection Sites
- Urinary tract, respiratory tract, gastrointestinal system, liver, and biliary tract are the most frequently affected sites in adults 2, 6
- Bloodstream infections and infective endocarditis occur rarely but carry poor prognosis 2, 4
- Intra-abdominal infections have been documented in patients with underlying conditions such as severe asthma requiring immunosuppressive therapy 1
- Osteomyelitis secondary to C. koseri is exceptionally rare 6
- Obstetric complications including preterm premature rupture of membranes (PPROM) and fetal loss have been reported 5
Antimicrobial Resistance Profile
- C. koseri demonstrates inherent resistance to ampicillin and emerging drug resistance to beta-lactams and aminoglycosides, often requiring carbapenem therapy 2
- As a member of Enterobacteriaceae, C. koseri can produce extended-spectrum beta-lactamases (ESBLs), conferring resistance to all penicillins, all cephalosporins, and aztreonam while maintaining susceptibility to carbapenems 3
- For serious infections, carbapenems (imipenem, meropenem, or ertapenem) remain the most reliable therapeutic option 3, 2
Treatment Considerations
- For infective endocarditis due to non-HACEK Gram-negative bacilli like C. koseri, combination antibiotic therapy with a beta-lactam (penicillins, cephalosporins, or carbapenems) plus either an aminoglycoside or fluoroquinolone for 6 weeks is recommended 2
- Given the emerging resistance patterns, carbapenem therapy is increasingly necessary for definitive treatment 2
- Gram-negative bacilli endocarditis carries poor clinical outcomes with higher rates of in-hospital mortality and complications, accounting for only 3-4% of all infective endocarditis cases 2
Clinical Pitfalls
- Clinicians should maintain heightened awareness for C. koseri infections in asthmatic patients requiring long-term oral corticosteroid administration, as these patients may develop concurrent infections with gut commensal bacteria 1
- The organism should be considered even in immunocompetent adults without traditional risk factors, as case reports increasingly document severe infections in this population 4, 6
- In nosocomial settings, C. koseri can be part of polymicrobial infections involving multiple Enterobacteriaceae species, requiring broad-spectrum empiric coverage 7, 3