Citrobacter koseri: Introduction, Morphology, Pathogenicity, Lab

Introduction

Table of Contents

Citrobacter koseri, formerly known as Citrobacter diversus, is a gram-negative bacterium belonging to the Enterobacteriaceae family. It is a facultative anaerobe, meaning it can grow both in the presence and absence of oxygen. The species name “koseri” is a tribute to the bacteriologist Leo Koser.

C. koseri is a rod-shaped bacterium, typically 1-3 micrometers long, and possesses flagella that enable it to move actively in liquid environments. It is a non-spore-forming bacterium.

This microorganism is widely distributed in the environment, including soil, water, and sewage. It can also be found in the human gastrointestinal tract as a normal commensal bacterium. However, under certain conditions, it can become an opportunistic pathogen and cause infections, especially in individuals with compromised immune systems or underlying health issues.

C. koseri is associated with a range of clinical infections, such as urinary tract infections (UTIs), respiratory tract infections, bloodstream infections (bacteremia), wound infections, and central nervous system infections, including meningitis in neonates and infants. It has also been linked to hospital-acquired infections and infections in intensive care unit (ICU) settings.

The pathogenicity of C. koseri is attributed to various factors, including its ability to produce certain virulence factors and the presence of specific adhesins that help it attach to and invade host cells.

In the clinical setting, C. koseri is usually identified through various laboratory tests, including microbiological cultures and molecular techniques. Treatment of C. koseri infections typically involves antibiotics, but antibiotic resistance can be a concern, making appropriate antibiotic selection crucial.

Morphology

Citrobacter koseri is a gram-negative bacterium with a distinct morphology. Under a microscope, it appears as a rod-shaped or bacillus bacterium. Here are some key morphological features of C. koseri:

Fig. Mucoid colony of Citrobacter koseri on CLED agar of urine specimen

Shape: The bacterium has a rod-like or bacillus shape. The rods are typically straight, elongated, and cylindrical.
Size: The average size of Citrobacter koseri cells is around 1-3 micrometers in length and 0.5-0.8 micrometers in width.
Arrangement: In microscopic preparations, Citrobacter koseri cells are often seen as single cells or arranged in short chains, but they can also occur in pairs or clusters.
Staining: When subjected to the Gram stain, Citrobacter koseri is classified as a gram-negative bacterium. It appears pink or red after staining, indicating that it does not retain the crystal violet stain and takes up the counterstain (safranin).
Motility: It is motile due to the presence of peritrichous flagella. These flagella are distributed around the entire cell surface, enabling the bacterium to move actively in liquid environments.
Capsule: It can produce a capsule, a protective layer outside the cell wall. The capsule aids in adherence to host cells and may play a role in evading the host’s immune system.

Pathogenicity

Citrobacter koseri is primarily considered an opportunistic pathogen, meaning it can cause infections in individuals with compromised immune systems or underlying health conditions. While it is a commensal bacterium commonly found in the gastrointestinal tract of humans and animals, it can become pathogenic under certain circumstances. Here are some factors that contribute to the pathogenicity of C. koseri:

Virulence Factors: Citrobacter koseri produces various virulence factors that enhance its ability to cause infections. These factors include adhesins that allow the bacterium to adhere to host cells, facilitating colonization and invasion. Additionally, it can produce toxins and other factors that contribute to tissue damage and inflammation.
Invasion and Spread: C. koseri can invade host tissues and spread from the initial site of infection to other parts of the body. This ability to disseminate within the host can lead to systemic infections, including bacteremia (presence of bacteria in the bloodstream).
Antibiotic Resistance: Some strains of Citrobacter koseri have developed resistance to commonly used antibiotics. This can complicate treatment and increase the severity of infections, especially in healthcare settings where antibiotic-resistant strains are more prevalent.
Vulnerable Populations: Its infections are more common in certain vulnerable populations, such as premature infants, neonates, elderly individuals, and immunocompromised patients. In neonates, C. koseri is a known cause of neonatal meningitis, a serious and potentially life-threatening infection of the central nervous system.
Nosocomial Infections: C. koseri can be responsible for hospital-acquired infections, particularly in intensive care units (ICUs). Factors like prolonged hospital stays, invasive medical procedures, and the use of medical devices can increase the risk of acquiring C. koseri infections in healthcare settings.

The clinical manifestations of Citrobacter koseri infections can vary depending on the site of infection. It is associated with a wide range of infections, including urinary tract infections (UTIs), respiratory tract infections, wound infections, intra-abdominal infections, and central nervous system infections like meningitis.

Lab Diagnosis

The laboratory diagnosis of Citrobacter koseri infections involves the isolation and identification of the bacterium from clinical samples. Here are the steps typically followed for the lab diagnosis of C. koseri:

Fig. Gram negative bacilli (GNBs) or Gram negative rods (GNRs) of Citrobacter koseri in Gram staining of cultute microscopy at a magnification of 4000X

Specimen Collection: A suitable clinical sample is collected from the site of infection suspected to be caused by Citrobacter koseri. Common samples include urine (for urinary tract infections), blood (for bacteremia), cerebrospinal fluid (for central nervous system infections like meningitis), wound swabs, respiratory secretions, and intra-abdominal fluids.
Microbiological Culture: The collected sample is streaked onto appropriate culture media, such as blood agar, MacConkey agar, and Mueller-Hinton agar. Citrobacter koseri is a gram-negative bacterium, so it will grow on media that support the growth of gram-negative organisms.
Incubation: The culture plates are incubated at the appropriate temperature (usually 37°C) for a specific period (usually 24-48 hours) to allow bacterial growth.
Colony Characteristics: After incubation, the colonies are examined for their morphology, size, shape, and color. Citrobacter koseri colonies typically appear as smooth, round, and moist colonies with a pale-yellowish color.
Gram Staining: A Gram stain is performed on the isolated colonies to determine their gram-negative nature. C. koseri should stain pink or red, indicating that it is a gram-negative bacterium.
Biochemical Tests: It can be differentiated from other closely related bacteria through a series of biochemical tests. Some common tests include Triple Sugar Iron (TSI) agar test, Indole production, Citrate utilization, Urea hydrolysis, and others. These tests help identify specific metabolic characteristics of Citrobacter koseri.
Serological Testing: In some cases, serological tests may be performed to identify specific antigens associated with Citrobacter koseri. These tests can aid in confirming the identity of the bacterium.
Antibiotic Susceptibility Testing: Once the bacterium is identified as Citrobacter koseri, it is important to determine its susceptibility to various antibiotics. This is crucial for guiding appropriate antibiotic therapy.
Molecular Testing: In modern laboratories, molecular techniques like polymerase chain reaction (PCR) may be employed for rapid and specific identification of C. koseri.

Treatment

The treatment of C. koseri infections involves the use of antibiotics to eliminate the bacteria and resolve the infection. However, the choice of antibiotics may depend on the site of infection, the severity of the infection, and the antibiotic susceptibility of the specific strain of Citrobacter koseri. It’s important to perform antibiotic susceptibility testing on the isolated strain to guide appropriate therapy, as some strains of Citrobacter koseri may be resistant to commonly used antibiotics.

Empirical Treatment: In severe infections or cases where the specific causative agent is unknown, initial treatment may involve broad-spectrum antibiotics that cover a wide range of gram-negative bacteria, including Citrobacter koseri. Commonly used empirical antibiotics include third-generation cephalosporins (e.g., ceftriaxone, ceftazidime) or carbapenems (e.g., meropenem, imipenem-cilastatin).

Definitive Treatment: Once the antibiotic susceptibility results are available, the antibiotic regimen can be adjusted to target the specific Citrobacter koseri strain. Preferred antibiotics may include:

Third-generation cephalosporins (e.g., cefotaxime, ceftriaxone) if the strain is susceptible.
Carbapenems (e.g., meropenem, imipenem-cilastatin) are effective against many C. koseri strains, including those resistant to cephalosporins.
Fluoroquinolones (e.g., ciprofloxacin) may be considered if the strain is susceptible, but their use should be reserved for specific situations due to concerns about increasing resistance.
Ampicillin-sulbactam and piperacillin-tazobactam are other possible treatment options.

Duration of Treatment: The duration of antibiotic therapy varies depending on the site and severity of the infection. In some cases, treatment may last for a few days to a week, while more severe infections, such as meningitis or bacteremia, may require a more extended course of treatment.

It’s important to complete the full course of antibiotics as prescribed by the healthcare provider, even if symptoms improve, to ensure complete eradication of the bacteria and prevent the development of antibiotic resistance.

Combination Therapy: In cases of severe infections or when dealing with antibiotic-resistant strains, combination therapy with multiple antibiotics may be considered. However, the use of combination therapy should be carefully evaluated, and it should be reserved for specific situations where the benefits outweigh the risks.

Prevention

Preventing Citrobacter koseri infections involves implementing measures to reduce the risk of exposure to the bacterium and controlling its spread. Here are some preventive strategies:

Hand Hygiene: Practicing proper hand hygiene is essential in preventing the transmission of C. koseri and other infectious agents. Regularly washing hands with soap and water or using alcohol-based hand sanitizers can help reduce the risk of infection.
Infection Control in Healthcare Settings: In hospitals and healthcare facilities, strict infection control measures should be implemented to prevent the transmission of C. koseri and other multidrug-resistant organisms. This includes appropriate cleaning and disinfection of medical equipment and patient care areas, proper handling and disposal of contaminated materials, and adherence to hand hygiene protocols by healthcare personnel.
Antibiotic Stewardship: Responsible and judicious use of antibiotics is crucial in preventing the development of antibiotic-resistant strains of Citrobacter koseri. Healthcare providers should follow appropriate guidelines for prescribing antibiotics, and patients should complete the full course of antibiotics as prescribed.
Proper Sterilization: Ensuring proper sterilization of medical instruments and equipment can help prevent infections caused by Citrobacter koseri during invasive procedures and surgeries.
Catheter Care: For patients with urinary catheters or other indwelling devices, proper catheter care and maintenance are essential to prevent urinary tract infections and other infections caused by C. koseri.
Infection Control in Neonatal Units: In neonatal intensive care units (NICUs), special attention should be given to infection control practices to prevent the transmission of C. koseri and other pathogens. Strict hand hygiene, isolation precautions for infected or colonized infants, and regular monitoring for outbreaks are important measures.
Food Safety: Practicing safe food handling and preparation can prevent foodborne infections, including those caused by Citrobacter koseri. Proper cooking, storage, and hygiene practices should be followed in both domestic and commercial food settings.
Promoting General Health: Maintaining a healthy immune system through a balanced diet, regular exercise, adequate rest, and managing underlying medical conditions can reduce the risk of opportunistic infections, including Citrobacter koseri.
Public Health Surveillance: Public health authorities should monitor and report cases of Citrobacter koseri infections to identify potential outbreaks and implement appropriate control measures.

Keynotes

Citrobacter koseri is a gram-negative bacterium belonging to the Enterobacteriaceae family. Here are some keynotes on C. koseri:

Gram-Negative Bacillus: It appears as a rod-shaped or bacillus bacterium under a microscope. It is classified as a gram-negative bacterium, which means it does not retain the crystal violet stain in the Gram stain and appears pink or red.
Facultative Anaerobe: C. koseri is a facultative anaerobe, capable of growing in both aerobic (presence of oxygen) and anaerobic (absence of oxygen) conditions.
Environmental Distribution: It is widely distributed in the environment, including soil, water, and sewage. It can also be found in the gastrointestinal tract of humans and animals as a normal commensal.
Opportunistic Pathogen: While typically a commensal, C. koseri can become an opportunistic pathogen, causing infections in individuals with weakened immune systems or underlying health conditions.
Infections: It is associated with various clinical infections, such as urinary tract infections (UTIs), respiratory tract infections, bloodstream infections (bacteremia), wound infections, and central nervous system infections, including neonatal meningitis.
Virulence Factors: The pathogenicity of C. koseri is attributed to various virulence factors, including adhesins that aid in adherence and invasion of host cells.
Motility: It is motile due to the presence of peritrichous flagella, which enable it to move actively in liquid environments.
Lab Diagnosis: Diagnosis involves the isolation and identification of the bacterium from clinical samples through microbiological culture, Gram staining, and biochemical tests. Molecular techniques may also be used for rapid and specific identification.
Antibiotic Treatment: Treatment of Citrobacter koseri infections typically involves antibiotics. The choice of antibiotics depends on antibiotic susceptibility testing results, and treatment may involve broad-spectrum antibiotics initially until specific susceptibility is determined.
Prevention: Preventive measures include proper hand hygiene, infection control in healthcare settings, antibiotic stewardship, proper sterilization of medical equipment, and promoting general health. In neonatal units, special attention to infection control is necessary.

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