Answer: Fusobacterium nucleatum. This patient suffered from septic arthritis of the right knee caused by Fusobacterium nucleatum. The bacteria caused extensive destruction of the femoral condyle's cartilage, with major inflammatory changes in the articulation of the patient, as shown in the MRI and the arthroscopic image.
Fusobacterium nucleatum is a fastidious anaerobic bacterium abundant in the oral cavity. It can cause various forms of periodontal diseases and infections of the head and neck, such as Lemierre's syndrome, mastoiditis, otitis, sinusitis, or retropharyngeal abscesses. Septic arthritis caused by Fusobacterium nucleatum rarely occurs in healthy children, and few cases have been reported (1, 2). Most case reports mentioned the presence of a risk factor, such as recent dental procedure, tonsillectomy, dental abscess drainage, or other pharyngeal infection, which was not the case in our patient.
This case highlights the role of anaerobic bacteria as a potential etiology in severe septic arthritis, even in a healthy child. Hence, anaerobic culture of synovial fluid should be considered for patients presenting with an inflamed joint and, more importantly, if nonresponding to first-line agents. Identification of an anaerobic bacterium in our patient allowed for the modification of the antibiotic regimen to ensure adequate anaerobic coverage. It must be noted that no anaerobic blood cultures were drawn in this patient and the joint fluid from the first arthrocentesis was not cultivated under anaerobic conditions. Our laboratory protocol indeed includes anaerobic cultures when the joint fluid is sent from the OR but not when the fluid is sent from the emergency department after a standard joint arthrocentesis. Following this case and our review of the literature, we modified our laboratory protocol for joint fluid, which includes anaerobic culture on every joint fluid aspirate if the quantity allows it.
For Fusobacterium infections specifically, the most commonly used empirical antibiotics include β-lactam agents, clindamycin, or metronidazole (3, 4). Some studies showed increasing β-lactamase production by Fusobacterium species, hence our initial choice of piperacillin-tazobactam (4). Agar dilution according to CLSI M11-A8 was performed after identification, which showed that the strain was susceptible to the following antibiotics: clindamycin, penicillin, cefoxitin, meropenem, metronidazole, and piperacillin-tazobactam. It is of interest to note that even with a susceptible microorganism, patients still showed signs of active disease after 4 weeks of treatment with piperacillin-tazobactam. This prompted our decision to add oral metronidazole, after which the patient showed a good clinical course.
See https://doi.org/10.1128/JCM.01262-17 in this issue for photo quiz case presentation.
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