Citrobacter koseri is a type of bacterium known as a gram-negative bacillus that belongs to the Enterobacteriaceae family. While often existing harmlessly, this microbe is recognized as a significant opportunistic pathogen that can cause severe, life-threatening infections in specific, vulnerable populations. Its presence signals a potential for serious illness, particularly when it gains access to parts of the body it does not typically inhabit.
The Microbe and Its Natural Environment
C. koseri is classified as a facultative anaerobe. This rod-shaped bacterium, formerly known as Citrobacter diversus, is motile, possessing peritrichous flagella. It is ubiquitous in the natural world, commonly isolated from environmental sources such as soil, water, and sewage systems.
The organism is also a common component of the commensal flora in the human gastrointestinal tract. In a healthy host, C. koseri typically remains confined to the gut without causing any adverse effects. However, it is an opportunistic pathogen. This transition from harmless resident to infectious agent is what defines its risk profile in clinical settings.
Primary Infections and Vulnerable Populations
The most concerning clinical manifestation of C. koseri infection is its ability to cause central nervous system (CNS) disease, specifically neonatal meningitis. This infection primarily targets newborn infants, especially those under two months of age. When C. koseri causes meningitis in neonates, it is frequently complicated by the formation of multiple brain abscesses.
Brain abscess formation occurs in up to 80% of reported neonatal C. koseri meningitis cases, which is significantly higher than the 10% rate associated with other gram-negative bacteria causing neonatal meningitis. This bacterium possesses a specific virulence factor, a 32-kD outer membrane protein, which is thought to enable it to cross the blood-brain barrier and invade the CNS tissue. Once in the brain, the organism can survive and multiply inside host cells like macrophages, which contributes to the establishment of chronic, difficult-to-treat abscesses.
While CNS infection is the most severe presentation, C. koseri can also cause other infections, particularly in immunocompromised individuals. These include urinary tract infections, pneumonia, and bloodstream infections. However, the neurological involvement in neonates remains the primary focus due to its high associated morbidity and mortality. Newborns, particularly those who are premature or have a low birth weight, represent the most vulnerable population because their developing immune systems and immature blood-brain barrier offer less resistance to the invading pathogen.
Transmission Routes and Risk Factors
Citrobacter koseri is transmitted through two principal routes: nosocomial acquisition and vertical transmission from mother to infant. Nosocomial infections occur in hospital settings, often in neonatal intensive care units (NICUs), where the bacteria can be spread through direct contact with contaminated medical equipment or via the hands of healthcare personnel.
Vertical transmission occurs when the infant acquires the bacteria from the mother, typically during birth. The bacteria can colonize the birth canal. Exposure during the passage through the birth canal can introduce the bacteria to the newborn, leading to colonization and subsequent potential for invasive infection.
The most prominent risk factors for CNS disease are those associated with neonatal vulnerability, such as prematurity and low birth weight. Beyond neonates, other groups at risk include elderly or debilitated patients, as well as individuals with weakened immune systems due to underlying medical conditions. Prolonged hospital stays, the presence of foreign bodies like urinary catheters, and recent surgical procedures also create opportunities for this opportunistic pathogen to cause infection.
Clinical Management and Long-Term Outcomes
The diagnosis of a C. koseri infection involves isolating and identifying the bacterium from clinical samples, such as blood cultures or cerebrospinal fluid (CSF) analysis. Early diagnosis is essential, especially in neonates presenting with fever or irritability, because the infection can progress rapidly to abscess formation. Neuroimaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), confirms the presence and extent of brain abscesses.
Treatment for severe C. koseri infections, particularly meningitis with abscesses, requires a prolonged course of intravenous antibiotics. The choice of antibiotic is complicated by the bacterium’s inherent resistance to certain drugs like ampicillin and the growing challenge of broader antibiotic resistance. Carbapenems, such as meropenem or imipenem, are often required due to their ability to penetrate the CNS and their effectiveness against resistant strains.
The presence of brain abscesses frequently necessitates surgical intervention to remove the pus and reduce pressure. Despite aggressive and appropriate treatment, the prognosis for neonatal C. koseri meningitis is serious, with mortality rates reported to be around 30%. Survivors face a high risk of long-term neurological sequelae, with up to 50% sustaining permanent damage to the central nervous system. These outcomes can include developmental delays, hemiparesis, seizures, and hydrocephalus.