The Gram-negative bacterium Citrobacter freundii belongs to the family Enterobacteriaceae and is increasingly recognized as a significant pathogen in healthcare settings. This rod-shaped, facultative anaerobic organism is often found in the environment and can colonize the human body without causing immediate harm. It becomes an opportunistic threat, primarily targeting patients with weakened immune systems or those receiving prolonged medical care. The major concern stems from its capacity to acquire multiple resistance mechanisms, leading to infections that are difficult to treat with common antibiotics.
Biological Characteristics and Clinical Significance
Citrobacter freundii is ubiquitous in nature, thriving in diverse environments such as soil, water, and sewage. This broad environmental presence allows it to easily enter the hospital environment, contaminating surfaces and medical equipment. The bacterium commonly resides in the intestinal tract of humans and animals as part of the normal gut flora.
The organism is classified as a healthcare-associated pathogen, meaning infections typically occur in hospitalized patients. The most vulnerable population includes the elderly, individuals in intensive care units (ICUs), and those with multiple underlying health conditions. Patients with indwelling medical devices, such as catheters or central venous lines, face an elevated risk of infection. The bacteria can form protective biofilms on these foreign materials.
Infections caused by C. freundii can be severe and affect various body sites. Frequent clinical manifestations include urinary tract infections (UTIs), bloodstream infections (sepsis), respiratory tract infections, and wound infections. CNS infection, leading to meningitis or brain abscesses, is particularly concerning, especially in neonates. Newborns are vulnerable due to their underdeveloped immune systems and the organism’s ability to cross the blood-brain barrier. Severe infections necessitate prompt and effective antimicrobial therapy.
Mechanisms of Antimicrobial Resistance
The clinical challenge posed by C. freundii is attributed to its highly adaptable mechanisms of antibiotic resistance. A fundamental trait is its intrinsic resistance, meaning it naturally possesses the gene for chromosomal AmpC beta-lactamase. This cephalosporinase enzyme hydrolyzes and inactivates many beta-lactam antibiotics, including penicillins and first- and second-generation cephalosporins.
The AmpC gene is regulated by the adjacent gene ampR, which controls enzyme production. In the absence of an inducing antibiotic, the AmpR protein acts as a repressor, keeping enzyme levels low. Exposure to certain beta-lactam drugs, however, can trigger a dramatic induction of AmpC production, leading to high-level resistance during treatment.
Mutations in the ampR gene can lead to derepression, causing the bacterium to constitutively overproduce AmpC, even without induction. This stable, high-level expression renders third-generation cephalosporins, like ceftriaxone, ineffective. Clinicians are then forced to use broader-spectrum agents. This intrinsic mechanism is distinct from acquired resistance, which C. freundii also readily develops.
Acquired resistance occurs via the horizontal transfer of resistance genes located on mobile genetic elements called plasmids. These plasmids carry genes for Extended-Spectrum Beta-Lactamases (ESBLs), such as CTX-M or SHV types. This broadens the resistance profile to include most extended-spectrum cephalosporins. The acquisition of carbapenemase genes transforms the organism into Carbapenem-Resistant Enterobacterales (CRE).
Common carbapenemases found in C. freundii include the Klebsiella pneumoniae Carbapenemase (KPC), the New Delhi Metallo-beta-lactamase (NDM), and OXA-48-like enzymes. These enzymes dismantle carbapenems, which were once considered the last line of defense against multidrug-resistant Gram-negative bacteria. High-level carbapenem resistance often results from the combination of a carbapenemase enzyme and the loss of outer membrane porin proteins. The loss of these porins restricts antibiotic entry, allowing the carbapenemase more time to neutralize the drug.
Laboratory Identification Methods
Identifying C. freundii infection begins with collecting clinical specimens, such as blood, urine, or respiratory secretions, depending on the infection site. These samples are cultured onto various selective and differential media in the clinical microbiology laboratory. MacConkey agar is routinely used, where C. freundii grows as a Gram-negative rod and often appears as a pink colony due to its ability to ferment lactose.
Initial species-level identification relies on a panel of biochemical tests. A characteristic trait of C. freundii is its positive utilization of citrate as a sole carbon source. It also produces hydrogen sulfide (H2S), visible as a black precipitate on specialized media. The organism is typically negative for the indole test and exhibits variable urease activity, requiring a combination of reactions for accurate identification.
Modern clinical laboratories expedite this process using automated systems like the VITEK-2, which analyzes biochemical reactions simultaneously. Molecular techniques, such as Polymerase Chain Reaction (PCR) targeting the 16S rRNA gene, are also employed for rapid species confirmation. The most important diagnostic step after identification is Antimicrobial Susceptibility Testing (AST). AST is performed using standardized methods, such as the Kirby-Bauer disk diffusion technique or broth microdilution (BMD), to determine effective antibiotics.
BMD is the gold standard, particularly for specialized drugs like colistin, as it accurately determines the minimum inhibitory concentration (MIC) required to stop bacterial growth. Specific phenotypic confirmation tests, such as the double-disk synergy test (DDST) for ESBLs and carbapenemase inhibition assays, confirm the presence of these resistance mechanisms.
Management and Prevention of Infection
Management of a C. freundii infection depends heavily on the source and the organism’s confirmed resistance profile. For severe, susceptible infections, carbapenems such as meropenem or imipenem are traditionally used as first-line therapy. Caution is exercised with third-generation cephalosporins or piperacillin-tazobactam due to the risk of AmpC induction leading to treatment failure.
If the organism is confirmed to be an ESBL producer, carbapenems remain the preferred choice for systemic infections, or non-beta-lactam antibiotics are selected based on AST. For less severe infections, such as an uncomplicated urinary tract infection, oral agents like nitrofurantoin or trimethoprim-sulfamethoxazole can be effective if the isolate is susceptible. The increasing prevalence of Carbapenem-Resistant C. freundii (CR-Cf) requires the use of novel or last-resort agents.
The therapeutic strategy for KPC-producing CR-Cf often involves newer beta-lactam/beta-lactamase inhibitor combinations, such as ceftazidime-avibactam. This drug is effective because its inhibitor component neutralizes both the KPC carbapenemase and the intrinsic AmpC enzyme. If the isolate is an NDM-producing strain, which is resistant to ceftazidime-avibactam, combination therapy using ceftazidime-avibactam paired with aztreonam is frequently utilized.
Infection control measures are paramount in preventing the spread of this opportunistic pathogen in healthcare settings. Strict adherence to hand hygiene protocols by all healthcare personnel is the single most impactful prevention strategy. Hospitals implement contact precautions, including single-patient rooms and the use of gowns and gloves, for patients infected with multidrug-resistant strains.
Active surveillance cultures screen high-risk patients, such as those newly admitted to the ICU, to identify asymptomatic carriers of CR-Cf. Prompt identification allows for early isolation and prevents patient-to-patient transmission. Hospitals also focus on antimicrobial stewardship programs to optimize antibiotic use, reducing the selection pressure that drives resistance. Careful management of indwelling medical devices, including prompt removal when no longer necessary, eliminates common sources of infection.