The genus Sphingomonas was divided into 4 clusters, and Sphingomonas yanoikuyae was renamed Sphingobium yanoikuyae (1). S. yanoikuyae is a gram-negative, nonsporulating, strictly aerobic rod-shaped bacterium (2) widely distributed in natural environments, especially in water and soil, and is rarely a human pathogen (3). Although 1 case of S. yanoikuyae infection has been reported in the central nervous system (CNS) of a child (4), infections have not been reported in adults. We report a case of S. yanoikuyae bacteremia in an older man.

An 89-year-old man from Japan sought care at an emergency department because of fever and chills lasting 1 hour. He had been taking prednisolone (5 mg/day) for 6 years for interstitial pneumonia. He was alert, and his vital signs were as follows: body temperature, 38.6°C; heart rate, 71 beats/min; blood pressure, 112/64 mmHg; respiratory rate, 28 breaths/min; and blood oxygen saturation, 100% while breathing room air. Laboratory findings revealed elevated leukocyte count (16,100 cells/μL; reference range 3,300–8,600 cells/μL) and C-reactive protein level (4.16 mg/dL; reference range 0–0.14 mg/dL) but were otherwise unremarkable. Chest computed tomography revealed honeycombing and multiple reticular shadows in both lungs, unchanged from 5 months earlier. We suspected sepsis and administered intravenous ceftriaxone (2 g/24 h) after obtaining 2 sets of blood samples for culture. On day 2, the patient’s fever subsided. On day 5, a blood culture sample yielded positive results after incubation in an aerobic BACTEC Plus Aerobic/F Culture Vial in a BACTEC FX system (Becton Dickinson, https://www.bd.com). Gram staining revealed small gram-negative rods (Figure, panel A) that we were unable to identify by using mass spectrometry (MALDI Biotyper; Bruker Daltonics, https://www.bruker.com). We subsequently cultured the positive blood culture fluid on Trypticase Soy Agar with 5% Sheep Blood (Becton Dickinson) at 35°C in an aerobic environment and identified S. yanoikuyae by using mass spectrometry of bacteria isolated on day 6 (Figure, panels B, C). Genetic analysis of a 1,402 nt 16S rRNA sequence revealed 99.5% homology with S. yanoikuyae (Appendix). We performed antimicrobial susceptibility testing by using the dilution method and a Neg MIC NF1J panel (Beckman Coulter, https://www.beckmancoulter.com) in accordance with Clinical and Laboratory Standards Institute (CLSI) criteria for other non-Enterobacterales bacteria (Table) (5). We determined the ceftriaxone MIC by using the Neg MIC EN 2J panel for Enterobacterales bacteria and Pos MIC 1J panel for gram-positive cocci (both Beckman Coulter). Although S. yanoikuyae was susceptible to ceftriaxone, we preferred to use antimicrobial drugs that were effective against glucose nonfermenting bacteria, which is the fermentation pattern exhibited by Sphingomonas spp. On day 6, we switched the antimicrobial to ceftazidime (1 g/8 h). We did not detect S. yanoikuyae in blood cultures at follow-up on days 6 and 11, indicating treatments were effective, and the patient’s condition remained stable. However, severe aspiration pneumonia developed on day 16, and he died of respiratory failure on day 17.

Within the genus Sphingomonas, S. paucimobilis is the most frequently reported cause of human infection (6), predominantly causing bacteremia, septicemia, peritonitis, lung infections, pneumonia, or urinary tract infections; 24 of 52 (46%) cases in published literature were of nosocomial origin (7). Thus, Sphingomonas spp. might be a chief cause of nosocomial infection in addition to other glucose nonfermenting bacteria. The S. yanoikuyae infection reported previously in a child was a nosocomial infection after head surgery (4). Although this case in an older man was not a nosocomial infection, he had been taking prednisolone for 6 years, which might have increased his infection risk.

No antimicrobial regimen has been established for treating S. yanoikuyae infections. The child who had a CNS infection received 28 days of intravenous meropenem and 5 days of intrathecal amikacin (4). A novel bacteria strain, CC4533, isolated from a contaminated Tris-acetate-phosphate agar plate used to grow Chlamydomonas reinhardtii, showed 99.55% DNA sequence identity to S. yanoikuyae; drug susceptibility testing indicated CC4533 was resistant to polymyxin B, penicillin, and chloramphenicol and sensitive to neomycin (8). We treated our patient with intravenous ceftriaxone and then ceftazidime. Cefepime, a 4th-generation cephalosporin, can penetrate the cerebral spinal fluid and has an additional quaternary ammonium group enabling penetration through the outer membrane of gram-negative bacteria, increasing effectiveness against β-lactamase–producing gram-negative bacilli (9). We selected ceftazidime, a 3rd-generation cephalosporin, because our clinical findings did not suggest a CNS infection, and S. yanoikuyae did not produce β-lactamase.

No breakpoints have been established for Sphingobium sp. bacteria; thus, we evaluated antimicrobial susceptibility according to CLSI criteria for other non-Enterobacterales bacteria (5). According to the dilution method, MIC values for ceftriaxone were >2 by using the Enterobacterales panel and <4 by using the gram-positive cocci panel. The ceftriaxone MIC for the isolate from this patient was 4, which is below the CLSI breakpoint of 8 for other non-Enterobacterales bacteria (5), indicating that the isolate was susceptible to ceftriaxone.

In conclusion, no standard antimicrobial treatment regimen has been established for S. yanoikuyae. Ceftriaxone and ceftazidime were effective treatments for S. yanoikuyae infection in this patient. Increased antimicrobial susceptibility data are needed to establish appropriate treatments for S. yanoikuyae.