Citrobacter freundii reaches humans through several distinct routes, with hospital environments serving as the single most important transmission setting. Once considered a mostly harmless environmental organism found in soil, water, and the intestines of healthy people, the bacterium has emerged as a significant opportunistic pathogen responsible for urinary tract infections, bloodstream infections, wound infections, and neonatal meningitis.1PubMed Central. The Citrobacter freundii Complex as an Emerging Pathogen: Genomic Plasticity, Virulence, and Antimicrobial Resistance Understanding how it travels from reservoirs to vulnerable patients is the foundation of preventing these infections, and the picture is more complex than simple person-to-person contact.
Hospital Plumbing as a Persistent Reservoir
One of the more troubling findings in recent years is that hospital wastewater systems act as long-term breeding grounds for C. freundii, including strains carrying genes for carbapenem resistance, which makes them extremely difficult to treat. A study tracing drug-resistant C. freundii clones through a hospital’s sanitary infrastructure found that these bacteria circulated in drains and plumbing for extended periods despite routine daily cleaning and periodic disinfection. The researchers proposed that the wastewater drainage system spread the organism between rooms and that sink drains served as launch pads for indirect transmission through bioaerosol plumes, meaning tiny droplets thrown into the air when water hits contaminated drain surfaces.2PubMed Central. Sanitary installations and wastewater plumbing as reservoir for the long-term circulation and transmission of carbapenemase producing Citrobacter freundii clones in a hospital setting – Section: CONCLUSIONS
This is not a minor footnote. Hospital plumbing is notoriously difficult to decontaminate because bacteria form biofilms, sticky colonies embedded in a protective matrix that clings to pipe walls and drain surfaces. Standard cleaning protocols often fail to penetrate these biofilms, allowing resistant clones to persist for months or years and seed new patient infections long after the original source patient has been discharged.
Healthcare Worker Hands and Shared Equipment
The classic route of hospital-acquired infection applies to C. freundii as well. Cross-contamination through the hands of healthcare workers, contaminated medical equipment, and devices like urinary catheters is a well-established pathway.3PubMed Central. Citrobacter: An emerging health care associated urinary pathogen One investigation of KPC-producing C. freundii spread across a healthcare system concluded that the likely transmission route was through healthcare workers’ hands contaminated by biofilm in a scrub sink or shared equipment in an interventional radiology suite. After targeted infection-prevention measures were implemented in that area, no additional patients with KPC-producing C. freundii were identified.4American Journal of Infection Control. Possible Transmission of Klebsiella Pneumoniae Carbapenemase (KPC)-Producing Citrobacter Freundii in a Healthcare System – Section: Conclusions
Indwelling devices deserve special mention. Any tube or catheter that bridges the outside world and a sterile internal site gives C. freundii a shortcut past the body’s natural barriers. Urinary catheters are the most common culprit: urinary specimens account for the largest share of clinical C. freundii isolates in hospital surveillance data.5The Journal of Infectious Diseases. Epidemiological and Molecular Surveillance of Multiresistant Citrobacter freundii Complex in a Tertiary Care Hospital: A Retrospective Cohort Study – Section: RESULTS Central venous lines and ventilator tubing create similar risks for bloodstream and respiratory infections.
Contaminated Food, Especially Raw Fish and Meat
Outside hospitals, food is a meaningful route of exposure. C. freundii is recognized as a foodborne contaminant in raw meat and fish products. A survey of local markets and butcher shops in Baghdad found the bacterium in roughly a third of carp fish samples, compared with lower rates in mutton and beef.6International Journal of Health Sciences. Isolation and molecular diagnosis of Citrobacter freundii in raw meat (beef, mutton and fish) in AL-Rusafa district of Baghdad city – Section: Abstract A separate study examining chicken and red meat in Iran similarly identified C. freundii as a zoonotic bacterium of public health importance, with many isolates carrying multidrug resistance genes.7PubMed Central. Unraveling Genetic Diversity and Antibiotic Resistance Mechanisms in Citrobacter freundii Isolates From Meat Products: Implications for Food Safety and Public Health
For most healthy people, ingesting a small number of C. freundii through undercooked food is unlikely to cause disease. The bacterium already lives harmlessly in many people’s intestines. The concern is twofold: first, foodborne exposure can silently colonize the gut with drug-resistant strains, which then sit quietly until the person becomes immunocompromised or needs antibiotics for something else. Second, in people who already have a weakened gut barrier, even food-grade doses can potentially lead to infection. Proper cooking, cold-chain maintenance, and basic kitchen hygiene dramatically reduce this risk.
Companion Animals
Pets represent another reservoir that most people do not think about. Dogs and cats can carry strains of C. freundii that produce extended-spectrum beta-lactamases, enzymes that break down many common antibiotics. These strains may also be resistant to fluoroquinolones, a class of antibiotics frequently used in human medicine. The risk of transmission from a pet to a healthy person is low, but for immunocompromised individuals living in close contact with a colonized animal, the exposure is more concerning.8International Journal of Antimicrobial Agents. Companion animals: a relevant source of extended-spectrum β-lactamase-producing fluoroquinolone-resistant Citrobacter freundii – Section: Abstract
The route here is straightforward contact: handling an animal, cleaning up after it, or sharing a living space where fecal particles can reach hands and eventually mouths. This is no different from how other gut-dwelling bacteria move between animals and humans, but it is worth flagging because veterinary antibiotic use can select for resistant C. freundii in pets, which then become a household source of resistant strains.
Mother-to-Newborn Transmission During Birth
Neonates face a unique transmission pathway: vertical transmission from a colonized mother during delivery. If the birth canal harbors C. freundii, the baby can acquire the organism as it passes through. When an infant develops symptoms within hours of birth, vertical transmission is the most likely explanation.9Pediatrics. Hospital-Acquired Citrobacter Meningitis Complicated by Pneumocephalus in a Neonate – Section: Discussion
Neonates are disproportionately vulnerable because their immune systems are immature and their gut microbiomes have not yet established the protective diversity that helps older children and adults resist colonization by potentially harmful bacteria. Neonatal intensive care units (NICUs) present additional hazards: multiple nosocomial outbreaks involving both C. freundii and the related species C. koseri have been documented in pediatric departments and NICUs.9Pediatrics. Hospital-Acquired Citrobacter Meningitis Complicated by Pneumocephalus in a Neonate – Section: Discussion In these settings, horizontal transmission through hands, equipment, and environmental contamination layers on top of any vertical exposure.
Drinking Water and Pipe Materials
Community drinking water systems are another potential, if often overlooked, exposure route. C. freundii can colonize biofilms inside water distribution infrastructure. Surveys of drinking water systems have repeatedly detected coliform bacteria, predominantly Citrobacter species, in water samples. Investigations traced the contamination not to general pipe surfaces but specifically to rubber-coated valves, which harbored visible biofilms containing the same coliform species found in the water downstream. These valves appeared to serve as point sources, seeding the water supply with bacteria despite adequate treatment at the plant.10PubMed. Contamination of drinking water by coliforms from biofilms grown on rubber-coated valves
Material matters. Experimental studies show that C. freundii survives for about a week on most common pipe materials like glass, stainless steel, PVC, and polyethylene, but rubber hoses allow it to persist and grow for many weeks.11PubMed. Microbial contamination of water by pipe and tube materials. 3. Behavior of E. coli, Citrobacter freundii and Klebsiella pneumoniae This finding has practical implications for building plumbing: rubber gaskets, flexible rubber connectors, and rubber-lined valves can become persistent colonization sites, especially in sections of a water system where flow is intermittent and chlorine residuals drop.
The Gut as Both Target and Staging Ground
The human intestinal tract is the most common site where C. freundii quietly resides without causing symptoms. In one hospital surveillance study, the majority of patients positive for multidrug-resistant C. freundii had intestinal carriage only, without any clinical infection.5The Journal of Infectious Diseases. Epidemiological and Molecular Surveillance of Multiresistant Citrobacter freundii Complex in a Tertiary Care Hospital: A Retrospective Cohort Study – Section: RESULTS Asymptomatic gut colonization matters because these carriers can shed the bacterium in their stool and serve as a source for environmental contamination and person-to-person spread in healthcare facilities.
Antibiotics disrupt this balance. By killing off the normal, protective gut bacteria, broad-spectrum antibiotics open ecological space for drug-resistant organisms like C. freundii to expand. The intestinal tract has long been recognized as a critical reservoir for antibiotic-resistant gram-negative bacteria, with the selective pressure of antibiotics playing a central role in allowing resistant strains to flourish and potentially spread to other patients.12Clinical Infectious Diseases. Antibiotic Regimens and Intestinal Colonization with Antibiotic-Resistant Gram-Negative Bacilli In a real sense, antibiotic exposure can convert a harmless bystander in the gut into a dominant, hard-to-treat pathogen ready to invade if the gut barrier is compromised.
Who Is Most Vulnerable
The populations at highest risk track closely with how transmission works. Hospital surveillance consistently shows that C. freundii infections are concentrated among older adults: in one tertiary hospital cohort, over half of identified cases were older than 65, and about six in ten were male. Most cases were hospital-acquired, with rehabilitation, geriatrics, and surgical departments seeing the heaviest burden.5The Journal of Infectious Diseases. Epidemiological and Molecular Surveillance of Multiresistant Citrobacter freundii Complex in a Tertiary Care Hospital: A Retrospective Cohort Study – Section: RESULTS A population-based study of bloodstream infections found a similar pattern: most episodes were healthcare-associated or nosocomial, and nearly half originated from the urinary tract. C. freundii complex bloodstream infections were more often linked to abdominal surgery and were more frequently polymicrobial compared with infections caused by the related species C. koseri.13PubMed Central. Citrobacter spp. bloodstream infection primarily affects the elderly either hospitalized or closely associated with health care – a population-based observational study with comparisons between C. koseri and the C. freundii complex – Section: RESULTS
Beyond age and hospitalization, specific underlying conditions amplify the risk. C. freundii infections cluster among people with diabetes, cancer, respiratory disease, and weakened immune systems, as well as newborns.14PubMed Central. A brief insight into Citrobacter species – a growing threat to public health – Section: Abstract The common thread is that anything degrading the immune system or the body’s physical barriers, whether a chronic disease, a surgical incision, a catheter, or simply being a premature infant, converts a commensal organism into a potential invader.
How Resistance Genes Spread Between Bacteria
One of the reasons C. freundii has become harder to treat is that transmission does not only happen at the level of the whole bacterium. Resistance genes travel between bacteria on mobile genetic elements called plasmids, small circular DNA molecules that can jump from one bacterium to another through a process called conjugation. This means a single drug-resistant C. freundii in a patient’s gut can hand off its resistance toolkit to other bacteria living alongside it, including species like E. coli and Klebsiella that are far more common causes of infection.
Research has shown this process is disturbingly efficient. One study identified C. freundii strains carrying three distinct plasmids encoding different carbapenemases, including a fusion plasmid that co-carried two separate carbapenem-resistance genes. All three plasmids could transfer to other bacteria, though at varying rates. The fusion plasmid showed a moderate but concerning ability to spread dual resistance genes simultaneously, raising the prospect that a single transfer event could render the recipient bacterium resistant to virtually all carbapenems, the antibiotics of last resort for serious gram-negative infections.15PubMed Central. High transconjugation efficiency of fusion plasmid pNDM_KPC in carbapenem-resistant Citrobacter freundii and its formation driven by IS26-mediated integration
Another study found a C. freundii isolate producing three different classes of carbapenemases, each encoded on a separate transferable plasmid. When tested, all three plasmids could transfer to E. coli at relatively high frequency, and the resulting bacteria inherited the corresponding resistance profiles.16Journal of Antimicrobial Chemotherapy. Citrobacter freundii resistant to novel β-lactamase inhibitor combinations and cefiderocol, co-producing class A, B and D carbapenemases encoded by transferable plasmids – Section: Results and discussion This makes C. freundii not just a pathogen in its own right but a kind of silent distributor of resistance genes within microbial communities, amplifying the threat far beyond the infections it directly causes.
Tracking Transmission With Molecular Tools
Figuring out exactly how C. freundii moved from point A to point B in a hospital used to be largely guesswork, relying on timing and proximity. Molecular surveillance has changed that picture. Whole-genome sequencing now allows researchers to compare the genomes of isolates from different patients and environmental samples at extremely fine resolution, distinguishing between bacteria that share a recent common ancestor and those that merely belong to the same species.
A retrospective study at a tertiary hospital combined genome sequencing with patient movement data to untangle how multidrug-resistant C. freundii complex spread over time. The results revealed that about a third of cases could be attributed to clonal transmission, meaning patients were infected by essentially the same bacterial strain, while roughly a fifth were linked through plasmid-mediated transmission, where different bacterial strains shared the same resistance plasmid. The remaining cases could not be linked to any identified transmission event.5The Journal of Infectious Diseases. Epidemiological and Molecular Surveillance of Multiresistant Citrobacter freundii Complex in a Tertiary Care Hospital: A Retrospective Cohort Study – Section: RESULTS The transmission clusters were sustained over long periods, with a median duration of about two years per cluster, confirming that once C. freundii gains a foothold in a hospital, it can circulate for extended stretches.
To make this kind of investigation more standardized and accessible, researchers have developed a core genome typing scheme for C. freundii covering over 3,000 target genes, validated against published outbreak datasets from hospitals in multiple countries. A companion scheme spanning several related Citrobacter species was also developed, enabling investigators to detect cross-species transmission events where resistance plasmids jump between different Citrobacter species within the same facility.17PubMed Central. Development and validation of a core genome multilocus sequence typing scheme for Citrobacter freundii: application in outbreak investigations and comparative analysis across the Citrobacter genus These tools are a significant step forward, but they require sequencing capacity that many hospitals around the world still lack.
What Actually Reduces Transmission
Given the variety of pathways, no single intervention can block all C. freundii transmission. In hospital settings, the evidence points to a layered strategy. Hand hygiene remains the most effective tool against direct contact spread, but it does nothing about plumbing reservoirs. Drain disinfection protocols, while helpful, often fail to eradicate biofilm-embedded bacteria in pipes. Some hospitals have experimented with replacing drain designs entirely, using waterless sinks or sealed waste systems in high-risk units.
For catheter-associated infections, minimizing the duration of catheter use and adhering to sterile insertion technique are the most impactful measures. Antibiotic stewardship plays a role that is easy to underestimate: by reducing unnecessary broad-spectrum antibiotic use, hospitals limit the selective pressure that allows resistant C. freundii to expand in patients’ guts and potentially spread to others.12Clinical Infectious Diseases. Antibiotic Regimens and Intestinal Colonization with Antibiotic-Resistant Gram-Negative Bacilli
Outside hospitals, reducing foodborne exposure comes down to thoroughly cooking meat and fish, maintaining cold-chain integrity, and preventing cross-contamination between raw and ready-to-eat foods. For drinking water, routine flushing of low-use taps and avoiding rubber components in plumbing where possible can reduce biofilm-related contamination. And for pet owners who are immunocompromised, basic hygiene after handling animals, particularly after cleaning litter boxes or picking up waste, provides a reasonable layer of protection against resistant strains that companion animals may carry.