Editor:
Citrobacter is a gram-negative coliform bacterial genus in the Enterobacteriaceae family, comprising 11 species well known for an ability to use citrate as their sole carbon source. They are generally considered organisms of low virulence and have only rarely been reported to cause serious human infections in immunocompromised patients. In particular, Citrobacter amalonaticus has seldom been isolated from humans.
In 2009, a 47-year-old Chinese woman was admitted to hospital for generalized abdominal pain and turbid dialysate fluid for 1 day. She had immunoglobulin A nephropathy with end-stage renal failure, and in 1998, she had undergone renal transplantation (living related donor), which was complicated by chronic graft rejection. She also had diabetes mellitus and was taking prednisolone and cyclosporine A because of the renal graft.
For the preceding 6 months, this patient had been regularly receiving intermittent peritoneal dialysis (IPD) because of worsening renal function. She was afebrile on admission. The total cell count in peritoneal effluent was 14 200×106/L. Gram stain of the effluent revealed the presence of numerous leukocytes, but no organisms were seen. A diagnosis of IPD-related peritonitis was made.
The patient was treated empirically with intraperitoneal cefazolin and tobramycin, but clinical response was poor. Culture of the effluent yielded pure growth of a motile, gram-negative rod, which was identified as C. amalonaticus based on biochemical tests and 16S rRNA gene sequencing (1) with 99.7% nucleotide identity. The isolate was susceptible to ceftriaxone, ceftazidime, piperacillin–tazobactam, imipenem, cotrimoxazole, amikacin, and gentamicin, but resistant to ampicillin, amoxicillin–clavulanate, cephalothin, and cefuroxime. After 2 days of the cefazolin and tobramycin, the patient’s antibiotic regimen was switched to intraperitoneal ceftazidime and amikacin for 1 week. The patient showed good response after the change of antibiotics, with a progressive drop in peritoneal fluid cell count to zero within 5 days. She was then transferred to another hospital for training in continuous ambulatory peritoneal dialysis. There has been no recurrence of peritonitis on follow-up for 2 years.
C. amalonaticus is a gram-negative, oxidase-negative, motile bacillus with peritrichous flagella. Other members of the Citrobacter species, such as C. freundii and C. koseri, are commonly found in water, soil, fish, animals, and food, but a possible environmental reservoir of C. amalonaticus is largely unknown, with only one report describing its isolation from catfish (2). C. amalonaticus is seldom isolated from humans; previous isolates were reported mainly from feces and occasionally from invasive infections, including enteric fever-like illness, osteomyelitis or arthritis, and polymicrobial abdominal abscesses (3–5). No dialysis-related peritonitis attributable to C. amalonaticus has previously been reported.
Citrobacter species are often resistant to first- and second-generation cephalosporins, and so third-generation cephalosporins or broad-spectrum beta-lactams such as carbapenems should be used in suspected Citrobacter peritonitis instead of the first-generation cephalosporins commonly used as first-line empiric therapy for dialysis-related peritonitis. Those suggestions accord with the facts in the present case, in which the patient did not initially respond to intraperitoneal cefazolin and tobramycin, but improved dramatically after a switch to ceftazidime and amikacin.
The bacterium probably gained entry to our patient’s peritoneum by transmural invasion from the gastrointestinal tract. Common micro-organisms causing dialysis-related peritonitis are those that can enter the abdominal cavity because of contamination during dialysate exchanges, pericatheter access from the exit site, and transluminal spread from the gastrointestinal tract or, less commonly, from a hematogenous source (6). Because C. amalonaticus is present in human feces, the gastrointestinal tract is most likely the primary source in our patient, who did not have symptoms of tunnel tract infection or systemic sepsis.
DISCLOSURES
The authors have no conflicts of interest to declare.
REFERENCES
- 1. Woo PC, Fung AM, Lau SK, Chan BY, Chiu SK, Teng JL, et al. Granulicatella adiacens and Abiotrophia defectiva bacteraemia characterized by 16S rRNA gene sequencing. J Med Microbiol 2003; 52:137–40 [DOI] [PubMed] [Google Scholar]
- 2. Nawaz M, Khan AA, Khan S, Sung K, Steele R. Isolation and characterization of tetracycline-resistant Citrobacter spp. from catfish. Food Microbiol 2008; 25:85–91 [DOI] [PubMed] [Google Scholar]
- 3. Suwansrinon K, Wilde H, Sitprija V, Hanvesakul R. Enteric fever-like illness caused by infection with Citrobacter amalonaticus. J Med Assoc Thai 2005; 88:837–40 [PubMed] [Google Scholar]
- 4. Freij BJ, Kusmiesz H, Shelton S, Nelson JD. Imipenem and cilastatin in acute osteomyelitis and suppurative arthritis. Therapy in infants and children. Am J Dis Child 1987; 141:335–42 [DOI] [PubMed] [Google Scholar]
- 5. Lipsky BA, Hook EW, 3rd, Smith AA, Plorde JJ. Citrobacter infections in humans: experience at the Seattle Veterans Administration Medical Center and a review of the literature. Rev Infect Dis 1980; 2:746–60 [DOI] [PubMed] [Google Scholar]
- 6. Yap DY, Lau SK, Lamb S, Choy BY, Chan TM, Lai KN, et al. An unusual organism for PD-related peritonitis: Hafnia alvei. Perit Dial Int 2010; 30:254–5 [DOI] [PubMed] [Google Scholar]