Janibacter melonis Bone Marrow Infection in an Immunocompetent Child With Prolonged Fever of Unknown Origin: A case report

Technical Summary: Janibacter melonis Bone Marrow Infection in an Immunocompetent Child

Problem Statement
Janibacter melonis is a rare environmental actinomycete previously documented in only two human cases, both presenting as bacteraemia in adults (one immunocompetent, one immunocompromised). There is no prior literature describing deep-seated bone marrow infections caused by this organism, nor any reports of such infections occurring in immunocompetent paediatric patients. This case addresses the diagnostic challenge of prolonged fever of unknown origin (FUO) in children when standard non-invasive investigations fail to identify an aetiological agent.

Methodology
The study presents a single-case report of a 13-year-old immunocompetent girl admitted with a 14-day productive cough and fever, initially diagnosed with pneumonia. The diagnostic approach followed a stepwise escalation:

1. Initial Management: Empirical treatment with amoxicillin-clavulanate, followed by doxycycline and methylprednisolone, provided only transient relief.
2. Exclusion of Common Pathogens: Extensive non-invasive workups, including nucleic acid amplification for Mycoplasma pneumoniae, serological surveys (bacterial, viral, fungal, autoimmune), and repeated blood cultures, yielded negative results.
3. Invasive Diagnostics: After the family declined bronchoscopy and next-generation sequencing (NGS), and the patient's condition persisted despite multiple antibiotic courses, a bone marrow aspiration and culture were performed.
4. Identification: The aspirate was cultured aerobically, yielding a slow-growing, cream-coloured colony. Identification was confirmed via matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) as Janibacter melonis.
5. Treatment Protocol: Guided by the limited existing literature on Janibacter species, the patient was treated with a combination of intravenous linezolid and trimethoprim-sulfamethoxazole. Standard antimicrobial susceptibility testing was not performed due to the lack of established breakpoints for this organism.

Key Results

• Microbiological Confirmation: Bone marrow culture successfully isolated J. melonis, while concurrent and subsequent blood cultures remained sterile, confirming a localized bone marrow infection rather than systemic bacteraemia.
• Clinical Response: The patient's fever resolved by day 6 of targeted combination therapy.
• Microbiological Clearance: A repeat bone marrow culture performed on day 20 of treatment was negative.
• Outcome: The patient completed a course of intravenous therapy followed by an oral regimen. At the three-month follow-up, the patient remained in sustained clinical remission with normalised inflammatory indices and no relapse.

Significance and Claims
The authors claim this report represents three distinct firsts in the medical literature:

1. The first documented case of J. melonis causing a bone marrow infection.
2. The first case of J. melonis infection occurring in an immunocompetent child.
3. The third overall human infection attributed to J. melonis.

The paper argues that this case expands the known clinical spectrum of J. melonis to include deep-seated infections in immunocompetent paediatric hosts. Furthermore, it supports the efficacy of a linezolid-based combination therapy (specifically with trimethoprim-sulfamethoxazole) for treating such infections.

Crucially, the authors propose a diagnostic shift for paediatric FUO: when standard non-invasive investigations are exhaustive and unrewarding, bone marrow culture should be considered early in the diagnostic algorithm. This procedure, while invasive, is noted to have a low complication rate and can detect occult pathogens, such as rare actinomycetes, that are missed by conventional sampling methods. The authors conclude that further reports are necessary to establish optimal treatment regimens and susceptibility breakpoints for this rare organism.