Citrobacter koseri (C. koseri) is a Gram-negative bacterium belonging to the Enterobacteriaceae family, which includes organisms like E. coli and Klebsiella. While often a harmless resident in the human gut, it is recognized as an opportunistic pathogen capable of causing serious infection. The danger it poses depends highly on the health status of the adult host and the site of infection. Understanding how C. koseri transitions from a benign colonizer to a threat is important, especially given its prevalence in hospitals and its emerging antibiotic resistance.
What is Citrobacter Koseri?
C. koseri, formerly known as Citrobacter diversus, is a motile, rod-shaped bacterium that thrives in both aerobic and anaerobic conditions. It is ubiquitous in the natural environment, found in soil, water, and sewage, and frequently colonizes the human gastrointestinal tract. In most healthy individuals, C. koseri exists as a commensal organism, living within the body without causing illness.
The organism transitions into a pathogen when it breaches the body’s natural barriers and enters normally sterile sites, such as the bloodstream or the urinary tract. This defines it as an opportunistic pathogen, unlike a primary pathogen that causes disease in a healthy host. Its capacity to utilize citrate as a sole carbon source and its specific biochemical profile aid in its identification in a clinical setting.
Factors Determining Severity in Adults
Risk Profiles
The severity of a C. koseri infection is determined by the patient’s underlying health and individual risk profile. The bacterium primarily targets medically fragile or immunocompromised adults, including those with poorly controlled diabetes, cancer, or chronic kidney disease. Adults over the age of 60 are also statistically more susceptible to severe Citrobacter infections.
Individuals with indwelling medical devices, such as urinary catheters or central venous lines, face an elevated risk because these devices allow bacteria to colonize and bypass mucosal defenses. In contrast, healthy adults rarely develop invasive C. koseri infections, and cases are typically localized and less severe. However, severe infections like infective endocarditis have been noted in adults without immunosuppression, indicating that risk is not entirely absent.
Common Clinical Manifestations
When C. koseri causes infection in adults, manifestations are typically localized to the genitourinary or respiratory tracts, or they can lead to systemic disease. The most common clinical syndrome is a Urinary Tract Infection (UTI), often complicated by catheter use or urinary tract abnormalities. C. koseri can also cause hospital-acquired pneumonia, particularly in patients on ventilators.
The most dangerous outcome is a bloodstream infection, known as bacteremia or sepsis, which is a systemic inflammatory response to the bacteria. Bacteremia often originates from an untreated UTI or an intra-abdominal source and is associated with significant morbidity and mortality, especially in the elderly. While central nervous system infections like meningitis are associated with C. koseri, they are far more common in neonates than in adults, though they can occur in adult neurosurgical patients.
Healthcare Association
A defining characteristic of C. koseri is its significant role as a nosocomial, or hospital-acquired, pathogen. The majority of adult bloodstream infections are associated with healthcare exposure, such as current hospitalization or recent contact with a medical facility. This association is partly due to the concentration of high-risk, medically compromised patients in these environments.
The hospital setting presents a higher likelihood of antibiotic exposure, which drives the selection and spread of drug-resistant strains. Consequently, healthcare-acquired infections are often more difficult to treat than those acquired in the community. The increasing incidence of C. koseri bloodstream infections is largely attributed to this high rate of healthcare-related infections.
Diagnosis and Management of C. Koseri Infections
Diagnosis
Diagnosis of a C. koseri infection relies on isolating the organism from a normally sterile body site, such as blood, urine, or wound fluid, using laboratory culture methods. Once identified, the next step is performing Antibiotic Susceptibility Testing (AST). AST determines which antibiotics will be effective against the specific strain causing the infection.
Identification of C. koseri in a sample confirms its pathogenic role and dictates the urgency of treatment. The organism’s biochemical profile, including its ability to ferment certain carbohydrates, helps distinguish it from other Gram-negative bacteria in the Enterobacteriaceae family. Molecular techniques like Polymerase Chain Reaction (PCR) are increasingly used to confirm identity rapidly.
Treatment Challenges
The major threat posed by C. koseri is its intrinsic and acquired resistance to antimicrobial agents, which complicates management, rather than its inherent virulence. The organism is naturally resistant to certain commonly used antibiotics, notably ampicillin. This intrinsic resistance means initial, broad-spectrum empiric therapy may fail if these agents are included.
A more concerning development is the bacterium’s capacity to acquire resistance mechanisms, such as the production of Extended-Spectrum Beta-Lactamases (ESBLs). ESBL-producing strains break down many types of penicillin and cephalosporin antibiotics, rendering them ineffective. While carbapenemase production is less common in C. koseri than in other Citrobacter species, carbapenem resistance is a growing global concern that further limits treatment options.
Treatment Protocols
Effective treatment for C. koseri infections must be tailored based on AST results, ensuring the chosen drug is active against the specific strain. For severe or systemic infections where resistance is suspected, empiric therapy often involves broad-spectrum agents until susceptibility results are available. Carbapenems, such as meropenem, are considered highly active against C. koseri isolates, including many ESBL-producing strains, and are often reserved for serious or resistant infections.
Other effective antibiotic classes include certain fluoroquinolones, aminoglycosides like amikacin, and trimethoprim/sulfamethoxazole. When the infection is localized, such as a simple UTI, a less broad-spectrum, susceptible agent may be used. Due to the organism’s prevalence in healthcare settings, infection control measures are also implemented to minimize transmission, including strict hand hygiene and sterilization protocols.