BONE AND JOINT INFECTIONS CAUSED BY FASTIDIOUS BACTERIA: WHICH BACTERIA ARE WE TALKING ABOUT?

Aim Microbiological investigations are essential for diagnosing bone and joint infections (BJIs). They enable definitive diagnosis by identifying the causative microorganism(s) and guide appropriate antibiotic or antifungal therapy. Culture remains the gold standard, but for fastidious or poorly cultivable organisms—due to intrinsic bacterial characteristics or prior antibiotic exposure—molecular techniques, notably broad-range 16S rRNA PCR, are relevant. As the laboratory of a National Reference Center for Complex BJI with a high volume of cases, we sought to quantify the frequency of such fastidious organisms and describe their detailed epidemiology. Methods We extracted electronic records of patients with positive broad-range 16S PCR results over two years (2024–2025). After selecting cases of BJI (biological matrices: synovial fluid, tissue biopsies, bone biopsies), we focused on identifications of bacteria considered uncommon in BJIs. Results A total of 613 patient records with BJIs and positive broad-range 16S PCR were retrieved for 2024–2025. In 60 cases (9.8%), sequencing identified anaerobic bacteria that had not been recovered by culture. In 48 cases (7.8%), sequencing yielded fastidious or uncommon bacteria: either non-cultivable on routine media (including Mycobacterium tuberculosis, Coxiella burnetii, Bartonella henselae, Borrelia spp, urogenital mycoplasmas, Legionella spp) or organisms potentially fragile on routine media and not isolated by conventional culture (e.g., Neisseria meningitidis, Neisseria gonorrhoeae, Campylobacter spp, Capnocytophaga canimorsus, Brucella melitensis, Kingella kingae, Globicatella sulfidifaciens). Conclusions Molecular diagnostics of microbiologically documented BJIs revealed a notably high proportion of anaerobic infections and a spectrum of sometimes unexpected pathogens that should be considered in diagnostic workups. Before excluding infection or concluding that etiologic investigations have failed, specialized diagnostic techniques must be employed, since organism identification is indispensable to tailor antimicrobial therapy and thereby influence functional prognosis. These findings support the need for multidisciplinary discussions among surgeons, microbiologists, and infectious disease specialists.