Citrobacter Freundii: Traits, Resistance, Diagnosis, and Care

Citrobacter freundii is a rod-shaped, gram-negative bacterium found widely in soil, water, and the human gut. Once dismissed as a harmless environmental organism, it has become a genuine concern in hospitals because of its built-in ability to resist many common antibiotics and its talent for picking up new resistance genes from other bacteria. The combination of increasing drug resistance and its capacity to cause serious infections in vulnerable patients puts it squarely on the radar of infectious-disease specialists worldwide.

Basic Traits and Taxonomy

C. freundii belongs to the family Enterobacteriaceae, the large group of bacteria that includes E. coli, Klebsiella, and Salmonella. It grows easily on standard laboratory media, ferments lactose (though sometimes slowly), and produces hydrogen sulfide, a feature that helps microbiologists tell it apart from close relatives. In a clinical laboratory study of Citrobacter isolates, C. freundii was the single most common species within the C. freundii complex, accounting for about 37% of isolates, followed by C. youngae at 24% and C. braakii at 13%.¹ These closely related species can be difficult to distinguish even with modern tools, which is why many labs report results at the complex level rather than pinpointing the exact species.

The organism is a natural inhabitant of the intestinal tract in a small percentage of healthy people, and it turns up regularly in environmental samples such as sewage, river water, and food. For most individuals with intact immune systems, carrying C. freundii in the gut causes no trouble at all. One study even found that higher gut abundance of C. freundii was associated with remaining free of colonization by drug-resistant E. coli during international travel, suggesting the organism can sometimes play a protective role in the gut ecosystem.² That protective role stands in sharp contrast to the havoc C. freundii can cause in hospitalized, immunocompromised patients.

What Makes It Dangerous

The C. freundii complex demonstrates striking genomic plasticity: it swaps DNA readily with neighboring bacteria through horizontal gene transfer, rapidly acquiring new virulence and resistance tools.³ Among the virulence factors researchers have cataloged, the Vi antigen stands out. In a study of isolates from ornamental fish, 65% carried the gene for Vi antigen production, a capsule-like structure that helps the bacterium dodge immune defenses. Classic toxin genes, by contrast, were rare, with heat-stable and heat-labile enterotoxin genes found in only about 10% and 5% of isolates, respectively.⁴

One especially alarming virulence trait is the bacterium’s ability to invade and replicate inside human brain microvascular endothelial cells. Laboratory work showed that C. freundii can cross the blood-brain barrier, get inside the cells lining brain blood vessels, and multiply there, a combination not seen with most other meningitis-causing bacteria.⁵ That intracellular hideout likely explains why Citrobacter meningitis in newborns so often leads to brain abscesses, a feature that sets it apart from more common causes of neonatal meningitis.

Infections in Adults

In adults, the infections C. freundii causes are overwhelmingly hospital-acquired. A study of bacteremia cases found that the most frequent source was intra-abdominal, accounting for about 61% of episodes. Urinary tract sources made up roughly 8%, intravascular catheter infections about 6%, and soft-tissue infections around 3%, with about 22% of cases having no identifiable source.⁶ Wound infections and respiratory tract infections also occur, particularly in patients with prolonged intensive-care stays, indwelling devices, or recent surgery.

Neonatal Meningitis and Brain Abscess

Citrobacter species are an uncommon cause of neonatal meningitis, but when they do cause it, the consequences can be severe. A majority of neonatal Citrobacter meningitis cases involve brain abscess formation, a complication far less common with other neonatal pathogens.⁷ Preterm infants are at highest risk. In at least one reported case, a preterm infant with C. freundii meningitis and brain abscess was successfully treated with a combination of antibiotics and surgical drainage, achieving normal neurological development at follow-up. But the intracellular replication mechanism described above means that antibiotics alone sometimes struggle to clear the infection, and outcomes are not always this favorable.

Who Is Most at Risk

Two overlapping but distinct risk profiles exist: one for acquiring C. freundii infection in general, and another specifically for acquiring a drug-resistant strain. For infection with ceftriaxone-resistant C. freundii in particular, the strongest predictors in one study were prior exposure to certain antibiotics, including broad-spectrum cephalosporins and imipenem, as well as staying in the hospital for a week or longer and spending time in an intensive care unit.⁸ For ceftriaxone-susceptible strains, risk factors skewed more toward underlying chronic conditions such as peripheral vascular disease, cerebrovascular disease, and AIDS.

A large case-control study from Denmark looking at carbapenemase-producing C. freundii confirmed that prolonged hospitalization and prior antibiotic exposure are the dominant risk factors. Hospital stays longer than two weeks, pancreatobiliary disorders, biliary or pancreatic surgery, and in-hospital exposure to vancomycin, fluoroquinolones, or beta-lactam combination antibiotics all carried strong associations.⁹ The pattern is consistent: the more antibiotics a hospitalized patient receives and the longer they stay, the greater the chance of encountering a resistant C. freundii.

Physical factors matter too. Invasive procedures, surgical operations, indwelling urinary catheters, and even frequent bed changes within a ward have been identified as potential risk factors during hospital outbreak investigations.¹⁰

The Built-In Resistance Problem

C. freundii carries a chromosomal AmpC beta-lactamase gene as standard equipment. Under normal conditions, the gene produces only low levels of the enzyme. But exposure to certain antibiotics, particularly third-generation cephalosporins, can switch on a regulatory protein called AmpR that dramatically ramps up production of AmpC. The result is that an infection initially susceptible to cephalosporins can become resistant during treatment itself.¹¹ This “inducible resistance” is what makes C. freundii, along with Enterobacter and Serratia, trickier to treat than garden-variety E. coli.

The clinical significance of the C. freundii complex lies less in unusual disease presentations and more in this moderate but real risk of resistance emerging during therapy with third-generation cephalosporins.³ It can catch clinicians off guard: susceptibility testing at the start of treatment says the drug should work, but resistance develops mid-course.

Carbapenem Resistance and Beyond

If AmpC-driven cephalosporin resistance is worrying, carbapenem resistance is the deeper fear. Carbapenems are typically the drugs of choice for serious AmpC-producing infections, so losing them narrows the treatment window substantially. Genomic surveillance in one European region found that the carbapenemase types in Citrobacter species broke down to roughly 42% KPC, 31% OXA-48-like, and 27% metallo-beta-lactamases such as VIM and NDM, with the resistance genes almost always sitting on transferable plasmids rather than the chromosome.¹²

The plasmid-based nature of these resistance genes is precisely what makes them so dangerous. Whole-genome sequencing of one Chinese C. freundii strain revealed 16 known resistance genes, including both KPC-2 and NDM-1 carbapenemases along with multiple extended-spectrum beta-lactamases and AmpC-type enzymes, conferring resistance to carbapenems, ceftazidime, and ciprofloxacin simultaneously.¹³ Other investigators found that KPC-2 and NDM-1 genes can sit on the same plasmid, meaning a single genetic transfer event could hand both carbapenemases to a new bacterium at once.¹⁴

In the worst-case scenario, a single isolate can carry class A, B, and D carbapenemases simultaneously. One such strain proved resistant to all beta-lactams, all three newer beta-lactamase inhibitor combinations (ceftazidime-avibactam, meropenem-vaborbactam, and imipenem-relebactam), and even cefiderocol, an antibiotic specifically designed to overcome many resistance mechanisms.¹⁵ Strains like this leave clinicians with essentially no first-line beta-lactam options.

How It Is Identified in the Lab

In most clinical laboratories today, identification relies on automated systems. MALDI-TOF mass spectrometry, which identifies bacteria by their protein fingerprint, has become the workhorse for bacterial identification. A multicenter study found that the VITEK MS system correctly identified about 97% of Enterobacteriaceae isolates, with roughly 84% identified to the species level and about 13% to the genus level.¹⁶ However, differentiating between closely related species within the C. freundii complex remains a challenge. In a head-to-head comparison, automated biochemical systems sometimes called a C. freundii isolate C. braakii while MALDI-TOF identified it as C. freundii, and vice versa.¹⁷

For resistance detection, the standard approach is antibiotic susceptibility testing on the cultured isolate. But because of the inducible AmpC issue, a modified double-disk test has been developed that can reliably distinguish between strains producing extended-spectrum beta-lactamases and those with derepressed chromosomal AmpC, a distinction that directly affects treatment choice.¹⁸ When carbapenem resistance is suspected, molecular testing for carbapenemase genes helps pin down the mechanism and guides infection-control responses.

Treatment Choices

Choosing the right antibiotic for C. freundii hinges on whether the strain carries only its native AmpC gene or has acquired additional resistance. For mild infections caused by susceptible strains, fluoroquinolones or trimethoprim-sulfamethoxazole can work. The real debate is over what to use for serious infections, especially bloodstream or deep-seated ones.

Cefepime, a fourth-generation cephalosporin, is more stable against AmpC enzymes than third-generation drugs and is often used as a carbapenem-sparing option. A retrospective cohort study of deep-seated infections caused by AmpC-producing Enterobacterales (including C. freundii) found unadjusted 30-day clinical failure rates of about 15% for cefepime and 11% for carbapenems. After statistical adjustment, carbapenem therapy was associated with lower odds of clinical failure.¹⁹ This suggests carbapenems remain the safer bet for serious infections when a carbapenem-susceptible AmpC producer is involved, though cefepime is not out of the picture entirely.

Prior antibiotic exposure matters for predicting trouble. Patients who had received antibiotics within 30 days before developing bacteremia, or who had been hospitalized within 90 days, were at higher risk for third-generation cephalosporin resistance. Transfer from another hospital or long-term care facility was a specific risk factor for non-susceptible strains.²⁰ Clinicians often use this kind of history to decide whether to start empiric therapy with a broader agent and then narrow once susceptibility results return.

Infection Control in Hospitals

Because resistant C. freundii spreads most readily in healthcare settings, infection-control measures are critical. One well-documented route of transmission is hospital plumbing. During an ICU outbreak of carbapenemase-producing Enterobacteriaceae, investigators traced the source to sink drains. Replacing the sinks and their plumbing, flushing them daily with a disinfectant solution, and reinforcing sink-use practices among staff ultimately brought the outbreak under control.²¹ Standard contact precautions, hand hygiene, and antibiotic stewardship programs that minimize unnecessary broad-spectrum antibiotic use remain the backbone of prevention.

C. freundii Outside the Hospital

The resistance problem with C. freundii does not stop at hospital walls. In the broader environment, the organism acts as both a reservoir and a courier for antibiotic resistance genes. Genomic analysis of Citrobacter from Australian wastewater and silver gull samples identified C. freundii strains carrying two carbapenemase genes simultaneously, along with resistance to up to nine classes of antibiotics.²² Wildlife like gulls can pick up resistant bacteria from wastewater and spread them over wide geographic areas.

Food animals are another important link. C. freundii isolated from domestic ducks in Bangladesh showed resistance rates ranging from 20% to 96% across eight commonly used antibiotics, with over 60% of isolates classified as multidrug-resistant and carrying resistance genes for beta-lactams, sulfonamides, tetracyclines, aminoglycosides, and fluoroquinolones.²³ Work from Egyptian livestock found even more alarming patterns, with many broiler chicken and buffalo isolates showing pan-drug-like resistance profiles and co-harboring multiple critical resistance genes.²⁴

Aquaculture adds yet another dimension. C. freundii has been documented causing disease in captive freshwater species including stingrays, frogs, peacock bass, and eels. Pathology in these animals included severe bacterial enteritis, ulcerative skin lesions, and liver and kidney damage.²⁵ Research has also shown that resistance plasmids harbored by C. freundii in fish guts can transfer horizontally to E. coli, and that common environmental chemicals like triclosan can accelerate this transfer.²⁶ This network of transmission between wastewater, wildlife, livestock, aquaculture, and human healthcare is why C. freundii resistance is increasingly viewed through a One Health lens.

Phage Therapy Research

With some strains now resistant to virtually all available antibiotics, researchers are exploring bacteriophages, viruses that specifically kill bacteria, as an alternative or complement to drugs. A phage called IME-JL8, isolated from sewage, proved highly effective against C. freundii in laboratory experiments, reducing bacterial counts by nearly seven log units within 20 minutes. In live fish experiments, administering the phage protected carp from lethal C. freundii infection and reduced inflammatory markers.²⁷

Phage therapy has even been attempted in a clinical veterinary setting. A loggerhead sea turtle with chronic, multidrug-resistant C. freundii skin disease received phage cocktails applied topically, intravenously, and into the exhibit water over eight months alongside conventional antibiotics. When C. freundii was subsequently detected again, it had reverted to normal antibiotic susceptibility patterns, suggesting the phage-antibiotic combination may have eliminated the resistant population.²⁸ Human clinical trials for phage therapy against Enterobacteriaceae are still in early stages, but cases like these provide proof of concept that phages can target C. freundii specifically and work alongside conventional drugs.