Citrobacter koseri is an opportunistic bacterium found globally and is a member of the family Enterobacteriaceae. While often a harmless inhabitant of the environment and the human body, this organism can transition into a pathogen under certain conditions. It is primarily recognized as a source of serious health risks within clinical settings, especially for susceptible patient populations.
Defining the Bacterium and Its Habitat
Citrobacter koseri is classified as a Gram-negative, rod-shaped bacterium. It is a facultative anaerobe, meaning it can grow and survive with or without oxygen, and it is motile due to the presence of peritrichous flagella. It belongs to the family Enterobacteriaceae, which includes many organisms commonly found in the intestines.
This microorganism has a wide environmental distribution, frequently isolated from soil, water, and sewage. It is also considered part of the normal flora of the mammalian intestinal tract, including humans and animals. The species was previously known as Citrobacter diversus and, in some contexts, was related to Levinea malonatica. In 1993, the species was formally designated as C. koseri.
Routes of Transmission and Vulnerable Populations
C. koseri causes infection primarily when acting as an opportunistic pathogen, entering a host with a compromised system or undergoing medical procedures. Transmission often occurs through the fecal-oral route, moving the organism from its natural intestinal reservoir to another host or site. The bacterium is also a common cause of hospital-acquired, or nosocomial, infections.
Within healthcare environments, the bacterium can spread horizontally through contaminated medical equipment, intravenous fluids, or direct contact with staff. Vertical transmission from the mother to the fetus is a possible route of infection in neonates. The organism poses a risk to patients hospitalized for more than 72 hours, as well as those with underlying chronic diseases like diabetes.
The most susceptible groups for severe, invasive disease include neonates, especially premature infants, and individuals who are immunocompromised. Neonates are at high risk due to their underdeveloped immune systems. Elderly patients are also vulnerable due to age-related decline in immune function and underlying health conditions.
Clinical Manifestations and Serious Infections
Infections caused by C. koseri range from relatively mild issues to life-threatening systemic disease. In adults, the bacterium causes urinary tract infections, pneumonia, and bloodstream infections, often in the context of other major health issues. Gastrointestinal issues may also occur, as the organism originates in the gut.
The most significant association of C. koseri is with central nervous system (CNS) infections in neonates and young infants. This bacterium is a leading cause of neonatal meningitis, an inflammation of the membranes surrounding the brain and spinal cord. A concerning feature is its strong tendency to cause brain abscesses, which are localized collections of pus within the brain tissue.
Brain abscesses occur in a high percentage of neonatal C. koseri meningitis cases, sometimes reaching 80%. Symptoms in infected neonates include fever, poor feeding, seizures, apnea, and a bulging fontanelle. These serious CNS infections carry a poor prognosis; mortality rates are high, and most survivors sustain significant neurological damage, such as hydrocephalus or intellectual delays.
Diagnosis and Therapeutic Strategies
Diagnosis of a suspected C. koseri infection begins with culturing the bacteria from clinical samples, such as blood, urine, or cerebrospinal fluid (CSF). Laboratory technicians use Gram staining to confirm the organism is a Gram-negative bacillus, followed by biochemical tests to identify the species.
Antibiotic susceptibility testing (AST) determines which antibiotics will be effective against the specific strain. Treatment relies on targeted antibiotic therapy, which is often complicated by the bacterium’s ability to develop resistance. C. koseri is known to produce resistance mechanisms, such as AmpC beta-lactamase, which can inactivate many common antibiotics.
Initial empiric treatment often involves broad-spectrum antibiotics, such as third-generation cephalosporins. The final drug choice, which may include carbapenems, must be guided by AST results. Infection control measures in hospitals, such as rigorous hand hygiene, are fundamental in preventing the spread of C. koseri. Early and aggressive treatment is necessary, especially in vulnerable neonates.