The Microbiome in Osteoarthritis: a Narrative Review of Recent Human and Animal Model Literature.

PURPOSE OF THE REVIEW: The microbiome has recently emerged as a powerful contributor to health and illness in chronic, systemic disorders. Furthermore, new microbiome niches beyond traditional gut locations are frequently being described. Over the past 5 years, numerous pivotal studies have demonstrated associations between changes in various microbiome niches and the development of osteoarthritis (OA). Herein, we review the most impactful recent literature, including microbiome associations with disease and the potential therapeutic value of microbiome manipulation. RECENT
FINDINGS: The gut microbiome of human OA patients is enriched in specific bacterial clades, most notably Streptococcus, which correlates with OA pain, Firmicutes, and others. Most studies have focused on knee OA, although one publication demonstrated positive associations with 3 gut microbiome clades in hand OA. OA can be easily distinguished from RA by evaluating differences in oral microbiome composition. Most studies have also demonstrated a reduction in richness of the gut microbiome (alpha diversity) associated with OA. Several studies have identified bacterial signatures within human knee and hip cartilage, synovial fluid, and synovial tissue and have described changes in these patterns occurring with the development of OA. In animal models of OA, high-fat diet-induced obesity has been the most well-studied OA risk factor associated with changes in the microbiome, with numerous bacterial clades changed within the gut microbiome and associated with OA. Also in animal models, various oral supplementations, including dietary fiber, probiotics including Lactobacillus species, and cecal microbiome transplantation have all shown improvements in OA histopathology or cartilage healing. Microbiome changes are strongly associated with the OA disease process and with individual OA risk factors related to both the gut microbiome and the microbial DNA patterns in the joint. Microbiome-directed interventions have the potential to prevent or reduce the progression of OA. Future studies should investigate the mechanistic underpinnings of these microbiome associations and further define the therapeutic potential of microbiome augmentation.