• Citrobacter koseri  is a gram-negative bacterium that belongs to the family Enterobacteriaceae.  Citrobacter koseri can cause various infections, including meningitis, pneumonia, urinary tract infections, and sepsis. 
• The epidemiology of  Citrobacter koseri  infections needs to be better characterized, as it is not a commonly reported pathogen. However, studies suggest that Citrobacter koseri infections are more common in neonates, elderly adults, and immunocompromised individuals. 
• Citrobacter koseri  infections are often hospital-acquired, and the risk factors for acquiring these infections include prolonged hospitalization, invasive procedures, and antibiotic use. Infections have been reported in various healthcare settings, including intensive care units, neonatal units, and long-term care facilities. 
• Citrobacter koseri  infections are typically treated with antibiotics, but some strains may resist certain antibiotics. Prevention of  Citrobacter koseri  infections in healthcare settings includes proper hand hygiene, appropriate use of antibiotics, and infection control measures such as isolation precautions when necessary. 

Structure and Classification 

Here is a brief overview of its structure and Classification: 

Structure: 

• Citrobacter koseri has a polysaccharide capsule that surrounds the cell wall and protects from environmental stressors and immune system responses. 

• The cell wall contains peptidoglycan, which gives the bacterium its rigidity. 

• It has flagella that allow for motility and pili that allow for attachment to surfaces and the exchange of genetic material. 

• Citrobacter koseri, like other Enterobacteriaceae, contains a single circular chromosome and plasmids that carry additional genetic information. 

Classification: 

Citrobacter koseri was first identified by a German bacteriologist, Hans Koser, in 1923 and was initially named Aerobacter aerogenes subsp koseri. 

Domain: Bacteria 

Phylum: Proteobacteria 

Class: Gammaproteobacteria 

Order: Enterobacterales 

Family: Enterobacteriaceae 

Genus: Citrobacter 

Species: koseri 

Citrobacter koseri  is an opportunistic pathogen and can cause various infections in humans, particularly in people with weak immune systems, such as those with HIV or those undergoing chemotherapy. 

• There are several antigenic types of Citrobacter koseri, which are classified based on their surface antigens. These antigens include the O antigen, the H antigen, and the K antigen. 
• The O antigen is a bacterial cell wall component, specifically the lipopolysaccharide (LPS) layer—several different O antigen types of Citrobacter koseri, including O1, O2, O3, etc. The H antigen is a flagellar antigen found on the bacterial flagellum—several different H antigen types of Citrobacter koseri, including H1, H2, H3, etc. Finally, the K antigen is a capsule, a polysaccharide layer surrounding the bacterial cell. There are several different K antigen types of Citrobacter koseri, including K1 and K2. 
• These antigenic types are essential for identifying and classifying Citrobacter koseri strains and developing vaccines and treatments for infections caused by this bacterium. 

The pathogenesis of  Citrobacter koseri is multifactorial and involves several virulence factors. Some of the essential virulence factors of  Citrobacter koseri  include: 

• Capsule: Citrobacter koseri produces a capsule that helps it evade the host immune system. The capsule comprises complex polysaccharides that make it difficult for phagocytes to engulf and kill the bacterium. 

• Adhesins: Citrobacter koseri expresses adhesins on its surface that help it attach to host cells. Adhesins are proteins that bind to specific receptors on the host cells, allowing the bacterium to colonize and invade host tissues. 

• Endotoxin: Citrobacter koseri produces endotoxin, also known as lipopolysaccharide (LPS), a significant component of its outer membrane. Endotoxin is a potent inducer of inflammation and can cause sepsis and septic shock. 

• Enterotoxin: Citrobacter koseri also produces an enterotoxin that can cause diarrhea and abdominal pain in infected individuals. 

• Iron acquisition: Citrobacter koseri has several mechanisms for acquiring iron, which is essential for bacterial growth and survival. These mechanisms include the production of siderophores and the expression of iron-binding proteins. 

The host defenses against Citrobacter koseri  include both innate and adaptive immune responses. 

Innate immune response: It is the first defense against  Citrobacter koseri. It includes the following mechanisms: 

• Physical barriers: The skin and mucosal surfaces act as physical barriers to prevent bacterial invasion. 

• Phagocytosis: Phagocytic cells, such as neutrophils and macrophages, engulf and destroy the bacteria. 

• Complement system: The complement system consists of proteins that can directly lyse or opsonize the bacteria, making them more susceptible to phagocytosis. 

• Cytokines: Cytokines are signaling molecules immune cells produce in response to bacterial infection. They can recruit more immune cells to the site of infection and activate them. 

The adaptive immune response is activated if the innate immune response is insufficient to eliminate the bacteria. It involves the following mechanisms: 

• T cells: T cells are a type of lymphocyte that can recognize and eliminate infected cells. They can also help activate other immune cells. 

• B cells: Can produce antibodies specific to  Citrobacter koseri. These antibodies can neutralize or opsonize the bacteria, making them more susceptible to phagocytosis. 

• Memory cells: After the infection is cleared, some T and B cells remain in the body as memory cells. These cells can quickly respond to future infection by  Citrobacter koseri. 

The clinical manifestations of  Citrobacter koseri infection can vary depending on the site of infection and the patient’s immune status. Here are some common clinical manifestations of Citrobacter koseri  infection: 

• Sepsis: Citrobacter koseri can cause sepsis, a potentially life-threatening condition characterized by fever, chills, rapid breathing, and low blood pressure. 
• Meningitis: Citrobacter koseri  is a known cause of meningitis, an infection of the membranes that surround the brain and spinal cord. Symptoms of meningitis may include headache, neck stiffness, fever, and confusion. 
• Pneumonia: Citrobacter koseri  can cause pneumonia, a lung infection that can cause coughing, chest pain, fever, and difficulty breathing. 
• Urinary tract infections: Citrobacter koseri  can cause urinary tract infections, which can cause Lower abdomen aches, frequent urination, and burning or pain while urinating. 
• Skin and soft tissue infections: Citrobacter koseri  can also cause soft tissue infections and skin, which can cause redness, swelling, pain, and warmth around the affected area. 

• The diagnosis of  Citrobacter koseri  infection usually involves a combination of clinical symptoms, laboratory tests, and microbiological analysis. Common signs and symptoms of a Citrobacter koseri  infection include fever, abdominal pain, diarrhea, and vomiting. 
• Laboratory tests can help to confirm the diagnosis. These tests may include urine tests, blood tests, and stool cultures. In addition, microbiological analysis can identify Citrobacter koseri  in samples collected from infected areas, such as wound or tissue samples. 
• The identification of  Citrobacter koseri  is usually done through biochemical tests. The bacterium is usually lactose-positive and produces a urease enzyme. It can also ferment glucose and other sugars. Identification can be further confirmed through molecular techniques, such as polymerase chain reaction (PCR) or sequencing of the bacterial DNA. 
• Treatment of  Citrobacter koseri  infections typically involves antibiotic therapy. However, the choice of antibiotic may depend on the severity of the infection, the location of the infection, and the susceptibility of the bacterium to different antibiotics. 

To prevent the spread of  Citrobacter koseri, the following measures can be taken: 

• Proper hand hygiene: One of the best ways to prevent the spread of Citrobacter koseri is to wash your hands in warm, soapy water consistently. It is essential after using the bathroom, changing a diaper, or preparing or eating food. 

• Proper food handling: Citrobacter koseri  can contaminate food incorrectly. Cook food thoroughly, refrigerate leftovers promptly, and avoid cross-contamination between raw and cooked foods. 

• Use of antibiotics: Citrobacter koseri  can be treated with antibiotics but using them correctly and only when necessary is essential. Resistance to antibiotics can result from overuse or improper handling of them. 

• Isolation precautions: Patients with Citrobacter koseri  infections may need to be isolated to prevent the spread of the bacteria to others. Healthcare workers should wear gloves and other personal protective equipment when caring for infected patients. 

• Environmental cleaning: Surfaces contaminated with Citrobacter koseri should be cleaned and disinfected regularly using appropriate disinfectants. 

• Citrobacter Koseri – an overview | ScienceDirect Topics 

• Comparative Genomic Analysis of Citrobacter and Key Genes Essential for the Pathogenicity of Citrobacter koseri – PMC (nih.gov)