Citrobacter amalonaticus is a Gram-negative bacterium belonging to the Enterobacteriaceae family. This organism is commonly found in various environmental settings and naturally exists within the intestinal tract of humans and animals. While usually a harmless resident of the gut, it can act as an opportunistic pathogen under certain circumstances. Its presence in a clinical setting often signals an underlying health vulnerability that allowed the bacteria to cause an infection.
Classification and Natural Reservoirs
Citrobacter amalonaticus is a rod-shaped, motile bacterium classified within the phylum Pseudomonadota and the class Gammaproteobacteria. It is a facultative anaerobe, meaning it can grow with or without oxygen. It was historically known as Levinea amalonatica.
The natural habitats of this species are widespread, making it a ubiquitous environmental organism. It is routinely isolated from sources like soil, water, and sewage, demonstrating its ability to survive outside of a host. In humans and animals, C. amalonaticus is considered a commensal organism, colonizing the gastrointestinal tract without causing disease.
Infection often occurs through translocation, where the bacterium moves from the gut to another site in the body. Its presence in a clinical sample, such as a wound or urine, usually indicates either an internal breach of the intestinal barrier or exposure to an external environmental source.
Clinical Significance and Disease Manifestations
C. amalonaticus causes a range of health problems, particularly in susceptible individuals. The most frequent site of infection is the urinary tract, where it is a common cause of urinary tract infections (UTIs). It is also isolated from wound and respiratory samples, especially in hospitalized patients.
Infections can escalate to severe, systemic conditions, including bloodstream infections (bacteremia or sepsis). Serious manifestations include central nervous system infections, such as neonatal meningitis. This risk is significant in newborns, where Citrobacter species can cause severe brain infections.
Rarer infections include gastroenteritis and an enteric fever-like illness resembling typhoid fever. Modern identification techniques, such as MALDI-TOF mass spectrometry, suggest that the prevalence of C. amalonaticus infections, particularly UTIs, may have been underestimated previously.
Patient Risk Factors and Vulnerable Populations
Infection with C. amalonaticus is strongly linked to a patient’s overall health status and exposure history. The organism typically causes disease only when the host’s normal defenses are weakened. Susceptible individuals are often those who are hospitalized, especially for prolonged periods, increasing exposure to hospital-acquired strains.
A compromised immune system is a major predisposing factor, including patients undergoing chemotherapy or those with HIV. Increased risk is also seen in people with significant underlying chronic diseases.
Chronic Conditions Increasing Risk
- Diabetes
- Various cancers (e.g., leukemia, pancreatic cancer)
- Chronic kidney disease, particularly in renal transplant recipients on immunosuppressive medication
Invasive medical procedures and devices provide a pathway for the bacteria to enter the body. The insertion of indwelling catheters or genitourinary instrumentation bypasses natural barriers, making the urinary tract vulnerable to colonization and infection. Both the very young and the elderly are vulnerable groups.
Medical Management and Antibiotic Resistance
Successful management of C. amalonaticus infections requires accurate identification and antimicrobial susceptibility testing. Because traditional biochemical tests can sometimes misidentify this species, laboratory identification often relies on advanced methods. Treatment is then tailored based on susceptibility results to determine effective antibiotics.
For severe infections, initial intravenous therapy often involves broad-spectrum agents such as third-generation cephalosporins, piperacillin-tazobactam, or carbapenems. As the patient improves, treatment may transition to oral options like fluoroquinolones (e.g., ciprofloxacin) or trimethoprim-sulfamethoxazole (TMP-SMX). The choice of drug is complicated by the organism’s increasing tendency toward resistance.
Multidrug resistance (MDR) is a growing concern, significantly limiting available treatment options. The bacteria produce various enzymes that break down antibiotics, including AmpC \(\beta\)-lactamases and Extended-Spectrum \(\beta\)-Lactamases (ESBLs). The presence of mobile resistance genes, such as those conferring resistance to carbapenems or colistin, requires careful monitoring and specialized antibiotic regimens.