Citrobacter koseri: Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes

Citrobacter koseri- Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes


Citrobacter koseri, formerly known as Citrobacter diversus, is a gram-negative bacterium that is a member of the Enterobacteriaceae family. It is an opportunistic pathogen capable of causing various infections, particularly in individuals with weakened immune systems or those who are hospitalized.

It is a facultative anaerobe, meaning it can survive and grow in both the presence and absence of oxygen. It is motile due to the presence of flagella, which enables it to move within its environment. The bacterium is non-spore-forming and has a rod-shaped morphology.

In terms of its habitat, C. koseri can be found in various natural environments such as soil, water, and the intestinal tracts of humans and animals. It is considered part of the normal gut flora, but it can also be present in the environment outside the body.

It is known to cause infections, especially in individuals with compromised immune systems. These include patients with underlying medical conditions, newborns, and individuals who have recently undergone surgery or medical procedures. The bacterium can enter the body through various routes, such as ingestion of contaminated food or water, inhalation of contaminated aerosols, or contact with contaminated surfaces or medical equipment.

Once inside the body, C. koseri can cause a range of infections depending on the site of entry. Common infections associated with this bacterium include urinary tract infections (UTIs), bloodstream infections (bacteremia), pneumonia, meningitis, and intra-abdominal infections. In newborns, it can lead to serious infections such as neonatal sepsis and meningitis.


Citrobacter koseri has a distinct morphology that helps in its identification. Here are the key features of its morphology:

  1. Shape: Citrobacter koseri is a rod-shaped bacterium, also known as a bacillus. The rods are typically straight or slightly curved.
  2. Size: The size of its cells can vary, but they generally range from 1 to 3 micrometers in length and about 0.5 to 0.8 micrometers in width.
  3. Gram staining: It is classified as a gram-negative bacterium. This means that during the Gram staining process, it does not retain the crystal violet dye and instead takes up the counterstain, which is usually safranin or fuchsin, giving it a pink or red color when observed under a microscope.
  4. Motility: It is motile due to the presence of flagella. These whip-like structures allow the bacterium to move in liquid environments, such as within culture media or the human body.
  5. Capsule: Some strains of its may produce a capsule, which is a protective layer outside the cell wall. The presence of a capsule can be observed using specific staining techniques or through the use of electron microscopy.


Citrobacter koseri is considered an opportunistic pathogen, meaning it typically causes infections in individuals with compromised immune systems or underlying health conditions. Here are some key points regarding the pathogenicity of Citrobacter koseri:

  1. Infection sites: It can cause infections in various sites of the body, including the urinary tract, bloodstream, respiratory tract, central nervous system, and intra-abdominal region. The specific infection depends on the route of entry and the patient’s susceptibility.
  2. Urinary tract infections (UTIs): It is one of the pathogens associated with UTIs. It can ascend the urinary tract and cause infections in the bladder, ureters, and kidneys. UTIs caused by it can present with symptoms such as frequent urination, burning sensation, cloudy urine, and lower abdominal pain.
  3. Bloodstream infections (bacteremia): It can enter the bloodstream and cause bacteremia. This can occur as a result of a primary infection at another site, such as a UTI, or through the introduction of the bacterium during invasive medical procedures or the use of contaminated medical devices. Citrobacter koseri bacteremia can lead to severe systemic illness.
  4. Respiratory tract infections: It can cause respiratory tract infections, such as pneumonia, particularly in hospitalized patients or those with pre-existing lung conditions. Ventilator-associated pneumonia is a concern in intensive care unit settings.
  5. Central nervous system infections: Citrobacter koseri has the ability to invade the central nervous system, leading to serious infections such as meningitis. It can be particularly problematic in newborns, where it is associated with neonatal meningitis, a potentially life-threatening condition.
  6. Intra-abdominal infections: Citrobacter koseri can cause infections in the intra-abdominal region, such as peritonitis or abscesses. These infections may arise from ruptured organs, surgical procedures, or other abdominal conditions.
  7. Antibiotic resistance: Iti has shown increasing antibiotic resistance, which can complicate treatment and contribute to the severity of infections. It is important to conduct antibiotic susceptibility testing to guide appropriate treatment choices.
  8. Virulence factors: It possesses various virulence factors that contribute to its pathogenicity. These include adhesins that allow it to attach to host tissues, enzymes that aid in tissue invasion and destruction, and factors that help it evade the host immune response.

Lab Diagnosis

The laboratory diagnosis of Citrobacter koseri infections typically involves the following steps and tests:

  1. Specimen collection: A suitable clinical specimen is collected based on the suspected site of infection. This can include blood, urine, cerebrospinal fluid, respiratory secretions, or intra-abdominal fluids, depending on the clinical presentation.
  2. Gram staining: A small portion of the specimen is smeared on a glass slide, heat-fixed, and stained using the Gram staining technique. It appears as gram-negative rods under the microscope.
  3. Culture: The specimen is inoculated onto appropriate culture media, such as blood agar, MacConkey agar, or other selective media that can support the growth of Enterobacteriaceae. Incubation is done at the appropriate temperature (usually 35-37°C) for 24 to 48 hours.
  4. Colony morphology: After incubation, characteristic colonies of Citrobacter koseri can be observed on the culture plates. They typically appear as large, smooth, gray or pale colonies. However, it’s important to note that colony appearance can vary among different strains.
  5. Biochemical tests: Various biochemical tests are performed to confirm the identification of C. koseri. These tests include oxidase test (negative), catalase test (positive), glucose fermentation, lactose fermentation, motility, indole production, urease activity, and other specific tests to distinguish Citrobacter koseri from closely related bacteria.
  6. Antibiotic susceptibility testing: Once the identification of Citrobacter koseri is confirmed, antibiotic susceptibility testing is performed to determine the appropriate antibiotics for treatment. The Kirby-Bauer disk diffusion method or automated systems like the VITEK or MicroScan can be used for this purpose.
  7. Molecular methods: In certain cases, molecular techniques such as polymerase chain reaction (PCR) or DNA sequencing may be employed for accurate identification and to detect specific virulence factors or antibiotic resistance genes.

It is important to follow proper laboratory safety protocols while handling clinical specimens and working with potentially infectious agents like C. koseri. Timely communication between the laboratory and the healthcare provider is crucial for prompt diagnosis and appropriate treatment of Citrobacter koseri infections.


The treatment of Citrobacter koseri infections typically involves the use of antibiotics. However, the choice of antibiotics should be guided by the susceptibility pattern of the specific strain and the site of infection. Here are some key points to consider regarding the treatment of C.  koseri infections:

  1. Antibiotic susceptibility testing: It is important to perform antibiotic susceptibility testing on the isolated strain of C .koseri to determine its susceptibility or resistance to different antibiotics. This helps in selecting the most effective antibiotic for treatment.
  2. Empirical therapy: In severe infections where specific susceptibility results are not yet available, empirical therapy may be initiated based on the known susceptibility patterns of Citrobacter koseri and other related bacteria. Commonly used empirical antibiotic regimens for Citrobacter koseri include combination therapy with broad-spectrum beta-lactam antibiotics (e.g., third-generation cephalosporins, carbapenems) along with an aminoglycoside or fluoroquinolone.
  3. Definitive therapy: Once the susceptibility results are available, the antibiotic regimen can be adjusted accordingly. The choice of antibiotic may be narrowed down to a more targeted agent based on the susceptibility profile.
  4. Combination therapy: In certain situations, combination therapy with two or more antibiotics may be necessary, especially for severe infections or when dealing with multidrug-resistant strains. The choice of combination therapy should be based on the susceptibility results and expert recommendations.
  5. Duration of treatment: The duration of treatment varies depending on the site and severity of infection. In general, treatment for Citrobacter koseri infections is typically continued for a minimum of 7 to 14 days. However, the duration may be extended in certain cases, such as infections involving the central nervous system or deep-seated abscesses.
  6. Follow-up testing: In cases of systemic infections or infections with complications, repeat cultures and tests may be needed to monitor the response to treatment and ensure eradication of the infection.

It is important to note that Citrobacter koseri has shown increasing antibiotic resistance, including resistance to commonly used antibiotics such as ampicillin, cephalosporins, and fluoroquinolones. Therefore, appropriate antibiotic selection should be based on local susceptibility patterns and expert recommendations, considering the patient’s clinical condition and risk factors.

In addition to antibiotic therapy, supportive measures and infection control practices are important for managing Citrobacter koseri infections effectively. It is crucial to consult with a healthcare professional for appropriate diagnosis and management of C. koseri infections.


Preventing Citrobacter koseri infections involves implementing a combination of infection control measures and general hygiene practices. Here are some key prevention strategies:

  1. Hand hygiene: Practicing good hand hygiene is essential in preventing the transmission of Citrobacter koseri and other infectious agents. Wash your hands thoroughly with soap and water for at least 20 seconds, especially before eating, after using the restroom, and after touching surfaces that may be contaminated. If soap and water are not available, use an alcohol-based hand sanitizer.
  2. Infection control in healthcare settings: Strict adherence to infection control protocols in healthcare facilities is crucial to prevent the spread of Citrobacter koseri infections. This includes proper hand hygiene, appropriate use of personal protective equipment (e.g., gloves, gowns, masks), and proper disinfection and sterilization of medical equipment.
  3. Environmental cleanliness: Regular cleaning and disinfection of surfaces and equipment in healthcare facilities and other settings can help reduce the risk of Citrobacter koseri contamination. Pay special attention to high-touch surfaces such as doorknobs, countertops, and shared equipment.
  4. Food safety: Practice safe food handling and preparation to prevent foodborne Citrobacter koseri infections. This includes proper washing of fruits and vegetables, cooking food thoroughly, avoiding cross-contamination between raw and cooked foods, and proper storage of leftovers.
  5. Water safety: Ensure the safety of drinking water by using reliable sources, treating water when necessary, and maintaining proper sanitation of water storage and distribution systems.
  6. Infection prevention in newborns: Implementing infection control measures in neonatal care units is crucial to prevent Citrobacter koseri infections in newborns. This includes strict hand hygiene, proper cleaning and disinfection of equipment, and appropriate isolation and cohorting of infected or colonized infants.
  7. Antibiotic stewardship: Responsible and judicious use of antibiotics helps prevent the emergence of antibiotic-resistant strains of Citrobacter koseri. Healthcare professionals should follow appropriate prescribing practices, including selecting the right antibiotic, using the correct dosage and duration, and avoiding unnecessary antibiotic use.
  8. Immunization: Vaccination against preventable infections, such as influenza and pneumococcal disease, can help reduce the risk of respiratory tract infections and subsequent complications caused by Citrobacter koseri.

It is important to note that the specific prevention strategies may vary depending on the setting and the individuals at risk. Consulting with healthcare professionals and following local guidelines and recommendations is crucial for effective prevention of C. koseri infections.


Here are some key points to remember about Citrobacter koseri:

  1. Citrobacter koseri is a gram-negative bacterium belonging to the Enterobacteriaceae family.
  2. It is an opportunistic pathogen that can cause infections, particularly in individuals with weakened immune systems or those who are hospitalized.
  3. It can be found in various natural environments such as soil, water, and the intestinal tracts of humans and animals.
  4. Common infections caused by Citrobacter koseri include urinary tract infections (UTIs), bloodstream infections (bacteremia), pneumonia, meningitis, and intra-abdominal infections.
  5. The bacterium is motile due to the presence of flagella and is non-spore-forming.
  6. It is typically diagnosed through laboratory tests such as Gram staining, culture, and biochemical tests to confirm its identification.
  7. Antibiotic susceptibility testing should be performed to guide appropriate antibiotic treatment, considering the strain’s susceptibility patterns and the site of infection.
  8. Citrobacter koseri has shown increasing antibiotic resistance, emphasizing the importance of prudent antibiotic use and surveillance of resistance patterns.
  9. Prevention of Citrobacter koseri infections involves practicing good hand hygiene, implementing infection control measures in healthcare settings, ensuring food and water safety, and promoting antibiotic stewardship.
  10. Timely diagnosis, appropriate treatment, and infection control measures are crucial for managing Citrobacter koseri infections effectively and reducing associated complications.

Further Readings

  1. “Clinical significance and antibiotic susceptibility patterns of Citrobacter koseri isolates” by Yayehyirad Kitaw et al. (2019) – This study explores the clinical significance and antibiotic susceptibility patterns of Citrobacter koseri isolates from patients in Ethiopia. It provides insights into the prevalence, clinical features, and antimicrobial resistance of this pathogen.
  2. “Citrobacter Infections: Epidemiology, Pathogenesis, and Management” by Zeeshan Afzal et al. (2017) – This comprehensive review article discusses the epidemiology, pathogenesis, clinical manifestations, and management of Citrobacter infections, including Citrobacter koseri. It covers various aspects, such as risk factors, diagnostic methods, treatment options, and emerging resistance patterns.
  3. “Citrobacter koseri infections in neonates: A comprehensive review” by Marisa Vidal et al. (2019) – This review article specifically focuses on Citrobacter koseri infections in neonates. It covers the epidemiology, risk factors, clinical manifestations, diagnosis, treatment, and outcomes associated with neonatal Citrobacter koseri infections.
  4. “Citrobacter koseri: An Unusual Cause of Neonatal Meningitis” by Mubashir Ahmed Khan et al. (2021) – This case report highlights a rare case of neonatal meningitis caused by Citrobacter koseri. It provides insights into the clinical presentation, diagnosis, and management of this specific infection.
  5. “Infection Control and Management of Citrobacter koseri in the NICU” by Sheba Pathrose et al. (2020) – This article focuses on the infection control and management strategies specific to Citrobacter koseri infections in the neonatal intensive care unit (NICU). It discusses preventive measures, diagnostics, treatment options, and outcomes.


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