Janibacter melonis bone marrow infection in an immunocompetent child with prolonged fever of unknown origin: a case report

Introduction Janibacter melonis ( J. melonis ) is a rare environmental actinomycete with only three previously reported human infections, all presenting as bacteraemia — all in adults, two of whom were immunocompetent and one of whom had refractory diffuse large B-cell lymphoma. Bone marrow involvement in an immunocompetent paediatric host has not been described. Case presentation A previously healthy 13-year-old girl presented with prolonged fever of unknown origin following an initial pneumonia-like illness. After extensive non-invasive investigations failed to identify an aetiological agent, bone marrow aspiration and culture identified J. melonis . She was successfully treated with a combination of intravenous linezolid and trimethoprim-sulfamethoxazole, achieving microbiological clearance by day 20 and sustained clinical remission at three-month follow-up. Conclusions This case expands the known clinical spectrum of J. melonis to include deep-seated bone marrow infection in immunocompetent children, and suggests that combination therapy with linezolid and trimethoprim-sulfamethoxazole may be a reasonable treatment option, although further evidence is required to define the optimal regimen. In selected clinical scenarios — such as when initial blood cultures remain negative or when advanced molecular diagnostics (e.g., metagenomic next-generation sequencing) are declined by the patient’s family — bone marrow puncture with extended culture incubation may be considered, particularly when infection by slow-growing or fastidious organisms is suspected. Prolonging the incubation period beyond the standard timeframe may enhance the recovery of rare and fastidious pathogens.