What the Research Says About the Gut Microbiome and Joint Disease

We only cite studies published in peer-reviewed journals. We summarize findings without overstating conclusions.

This article summarizes a state-of-the-art narrative review published on March 13, 2024, in the International Journal of Molecular Sciences (MDPI), titled “Role of the Gut Microbiota in Osteoarthritis, Rheumatoid Arthritis, and Spondylarthritis: An Update on the Gut–Joint Axis.” The review was authored by a nine-member international team spanning six institutions: Umile Giuseppe Longo, Alberto Lalli, and Benedetta Bandini (Research Unit of Orthopaedic and Trauma Surgery, Fondazione Policlinico Universitario Campus Bio-Medico and Università Campus Bio-Medico di Roma, Rome, Italy); Roberto de Sire (Gastroenterology, Endoscopy Unit, IRCCS Humanitas Research Hospital, Rozzano, and the Gastroenterology Unit, University Federico II of Naples, Naples, Italy); Silvia Angeletti (Unit of Clinical Laboratory Science, University Campus Bio-Medico of Rome); Sebastien Lustig (Orthopaedic Department, Lyon North University Hospital, Hôpital de La Croix Rousse, Hospices Civils de Lyon, and Claude Bernard University Lyon 1, Lyon, France); Antonio Ammendolia and Alessandro de Sire (Department of Medical and Surgical Sciences and Research Center on Musculoskeletal Health, University of Catanzaro “Magna Graecia,” Catanzaro, Italy); and Nicolaas Cyrillus Budhiparama (Department of Orthopaedics, Leiden University Medical Center, Leiden, The Netherlands). The review synthesizes current scientific understanding of the gut-joint axis – the biological communication pathway between the intestinal microbiome and joint tissues – and examines how disruptions in that relationship may contribute to the onset and progression of osteoarthritis, rheumatoid arthritis, and spondylarthritis. It further reviews validated outcome measurement tools used in arthritis research and summarizes emerging microbiota-targeted treatment strategies. The full text is freely available at PubMed Central (PMC10970477).

The authors declare no conflicts of interest. No external funding was reported for this review.

Background: Your Gut Microbiome Is Not Just a Digestive Tool

The human gut is home to an extraordinary community of microorganisms – bacteria, fungi, viruses, and archaea – collectively known as the gut microbiota. A healthy adult carries somewhere in the range of 38 trillion bacterial cells in the gastrointestinal tract, representing hundreds to thousands of different species. For most of medical history, these organisms were understood primarily as participants in digestion and nutrient absorption. That understanding has been fundamentally revised over the past two decades.

It is now clear that the gut microbiota functions as a dynamic regulatory system with far-reaching effects on human physiology: it trains and calibrates the immune system, synthesizes essential vitamins and signaling molecules, regulates metabolic processes, communicates with the nervous system through what is called the gut-brain axis, and maintains the structural integrity of the intestinal lining that separates the microbial world of the gut from the bloodstream. When this ecosystem falls into disarray – a state called dysbiosis, meaning an imbalance in the composition, diversity, or function of the microbial community – the consequences extend well beyond the gut itself.

Research over the last decade has accumulated substantial evidence that gut dysbiosis contributes to the development and worsening of diseases far removed from the intestine: metabolic conditions like obesity and type 2 diabetes, neurological conditions like depression and multiple sclerosis, cardiovascular disease, and inflammatory bowel disease. The emerging chapter of this story involves the joints. This review by Longo, de Sire, and colleagues brings together the current state of evidence for what researchers now call the gut-joint axis – the bidirectional biological link between the gut microbiome and joint health.

The Gut-Joint Axis: How Intestinal Bacteria Influence Distant Joints

The concept of the gut-joint axis rests on several interconnected biological mechanisms by which events in the gut can drive inflammation in joint tissue. The authors identify three primary pathways.

The Leaky Gut and Bacterial Translocation

The interior surface of the intestine – the gut mucosa – is lined by a single layer of epithelial cells joined together by proteins called tight junctions. This barrier normally allows nutrients, water, and small molecules to pass into the bloodstream while physically blocking bacteria and their products from doing so. When the gut microbiota is healthy and diverse, it actively supports the integrity of this barrier: certain bacteria produce compounds that strengthen the tight junctions and maintain the mucus layer that sits above the epithelium as a first line of defense.

In dysbiosis, this protective function degrades. The barrier becomes more permeable – colloquially described as “leaky gut” – allowing bacterial components that would normally stay contained in the intestinal lumen to enter the systemic circulation. The most important of these components is lipopolysaccharide (LPS), a structural molecule that forms part of the outer membrane of a large class of bacteria called Gram-negative bacteria. LPS is a potent trigger of the innate immune system: when immune cells detect it in the bloodstream, they respond with a cascade of inflammatory signaling molecules including tumor necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6). Research cited by the authors from a 2017 study showed that LPS from gut-resident bacteria activates this cytokine cascade and drives T-cell-mediated pathogenesis of arthritis. The resulting low-grade systemic inflammation, sustained by ongoing bacterial translocation from a persistently leaky gut, reaches joint tissues and contributes to synovial inflammation, cartilage breakdown, and bone erosion.

Molecular Mimicry and Autoimmunity

A second mechanism involves a phenomenon called molecular mimicry. The immune system learns to distinguish “self” from “foreign” through the shapes of molecules – primarily proteins – that it encounters throughout development. Bacteria carry surface proteins and fragments that the immune system must learn to recognize as foreign without triggering destructive cross-reactions against the body’s own tissues. The problem arises when bacterial proteins happen to share structural similarity with proteins found in the host’s own cells or tissues. When the immune system mounts a response against such a bacterium, the antibodies and T cells it generates may also mistakenly attack host tissue bearing a similar molecular signature.

The authors note that up to 30 percent of the composition of the gut microbiome is influenced by hereditary factors. In individuals with specific genetic predispositions – including those carrying particular variants of immune regulatory genes – this molecular mimicry may be especially likely to trigger autoreactive immune responses. The bacterial phyla Firmicutes and Proteobacteria have been specifically associated with this mechanism in the context of arthritis susceptibility. In essence, a susceptible person’s gut bacteria may inadvertently train their immune system to attack their own joint tissues.

Disruption of the Th17/Treg Balance

The third mechanism involves the gut microbiome’s central role in calibrating the balance between two functionally opposing classes of immune cells: T-helper 17 (Th17) cells and regulatory T cells (Tregs). Th17 cells are pro-inflammatory effectors that produce interleukin-17 (IL-17), a potent driver of joint inflammation implicated in both rheumatoid arthritis and spondylarthritis. Tregs, by contrast, suppress excessive immune responses and prevent autoimmunity. A healthy microbiome actively promotes Treg development and keeps Th17 activity in check. In dysbiosis, this balance shifts: Treg populations contract, Th17 cells expand, and the immune system becomes predisposed to the kind of persistent inflammatory signaling that damages joints. The authors note that shifts in the Th17/Treg ratio are a recognized feature of several arthritis conditions, and that correcting gut dysbiosis can partially restore this balance – a finding with direct therapeutic implications.

The Gut Microbiome in Osteoarthritis

Osteoarthritis affects more than 25 percent of adults over age 18 and afflicted an estimated 303 million people worldwide in 2017. It was long understood primarily as a mechanical disease – the inevitable consequence of decades of joint loading wearing down cartilage – but that framing has been substantially revised. OA is now recognized as a complex disease in which chronic low-grade inflammation plays a central and underappreciated role alongside the mechanical factors.

The gut microbiome connection to OA operates through this inflammatory dimension. Research reviewed by the authors shows that OA patients demonstrate characteristic alterations in their gut bacterial composition compared to healthy individuals: elevated levels of the phyla Actinobacteriota and Proteobacteria alongside reduced levels of Firmicutes. This pattern is consistent with a pro-inflammatory gut environment, because several important anti-inflammatory, short-chain fatty acid-producing bacteria belong to the Firmicutes phylum.

The proposed sequence is as follows: dysbiosis leads to increased gut permeability; LPS and other bacterial products enter systemic circulation; this drives low-grade systemic inflammation; inflammatory mediators reach joint tissue and act on chondrocytes (cartilage cells), synovial fibroblasts, and subchondral bone cells; and these cells respond by producing more degradative enzymes and inflammatory cytokines, accelerating cartilage loss and synovial inflammation. Additionally, the fat pad of the knee joint (the infrapatellar fat pad) participates in this process by secreting inflammatory cytokines, adipokines, and growth factors in response to the local inflammatory environment – a local amplification loop that may sustain joint inflammation independently of ongoing gut signals.

At the molecular level, the authors review several key signaling pathways involved in OA development that intersect with microbiome-mediated inflammation. Transforming Growth Factor-beta (TGF-β) normally protects cartilage by preventing chondrocytes from transitioning into a hypertrophic, destructive state; when TGF-β signaling is disrupted – as occurs in the context of chronic inflammation – chondrocyte hypertrophy and cartilage deterioration follow. The Wnt/β-catenin and Indian Hedgehog (Ihh) signaling pathways, which normally regulate the balance between cartilage synthesis and breakdown, are also destabilized by inflammatory mediators that gut dysbiosis can sustain. IL-6, which is secreted by activated fibroblasts in the synovial membrane, is independently associated with both OA pain severity and radiographic disease progression – and IL-6 production is directly driven by the pro-inflammatory cytokine cascade that LPS triggers.

The Gut Microbiome in Rheumatoid Arthritis

Rheumatoid arthritis differs from osteoarthritis in being a systemic autoimmune disease rather than a primarily degenerative one. In RA, the immune system mounts a sustained attack against the synovial lining of joints, producing the characteristic pattern of symmetric polyarthritis – typically affecting the small joints of the hands and feet, including the wrists, the knuckle joints (metacarpophalangeal and metatarsophalangeal joints), and the proximal finger joints. Left undertreated, RA causes progressive joint destruction, deformity, and loss of function.

The evidence connecting gut dysbiosis to RA is among the strongest in the gut-joint axis literature, and it is notable for a key temporal detail: changes in the gut microbiome are detectable in the preclinical stages of RA, before joints are clinically affected. This preclinical dysbiosis – documented in multiple recent studies – implies that gut microbial disruption is not merely a consequence of having RA (due to medications, reduced activity, or systemic inflammation altering the gut environment) but may actually precede and contribute to the disease’s onset.

Specific bacterial genera have been identified in this dysbiotic pattern. A 2019 study examined the gut microbiome of female RA patients and found that the phylum Bacteroidetes was elevated in early RA patients while Actinobacteria (particularly the genus Collinsella) were enriched in healthy controls – suggesting a loss of protective bacterial populations and enrichment of potentially pro-inflammatory ones in RA. Multiple other studies have detected DNA from bacterial species including Prevotella, Fusobacterium, Porphyromonas, and Bacteroides not only in the stool of RA patients, but in their serum and synovial fluid – the fluid inside the joint cavity. The presence of bacterial genetic material in joint fluid is a striking finding: it suggests that bacteria or their fragments may physically migrate from the gut to joint tissue, where they could activate local immune responses and sustain inflammation.

The pathogenesis of RA involves a combination of genetic susceptibility and environmental triggers. On the genetic side, the HLA-DRB1 locus – a gene that controls how the immune system recognizes and responds to foreign proteins – is the strongest known genetic risk factor for RA, and its association with the disease highlights T-cell-mediated immune responses as central to RA pathogenesis. The gut microbiome appears to function as one of the key environmental triggers: by generating molecular mimicry signals, disrupting Treg/Th17 balance, and sustaining systemic inflammatory tone through LPS translocation, dysbiotic gut communities may initiate the mucosal autoimmunity that eventually spreads to the synovial joints.

At the cellular level, RA is characterized by the production of two classes of autoantibodies – rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPAs, also called anti-CCP antibodies) – whose presence years before joint symptoms appear further supports the idea that systemic autoimmune processes precede clinical disease. Gut bacteria produce enzymes that citrullinate proteins (convert arginine residues to citrulline, a chemical modification the immune system can recognize as foreign in susceptible individuals), potentially initiating ACPA production at mucosal sites before the autoimmune response spreads to joints.

The Gut Microbiome in Spondylarthritis

Spondylarthritis (SpA) is not a single disease but a family of related inflammatory conditions united by their tendency to affect the axial skeleton (spine and sacroiliac joints), entheses (the sites where tendons and ligaments attach to bone), and peripheral joints. The major subtypes include ankylosing spondylitis (the most common and best-studied form), psoriatic arthritis, reactive arthritis (which by definition follows gastrointestinal or urogenital infections), arthritis associated with inflammatory bowel disease, and a subgroup of juvenile idiopathic arthritis.

The gut connection to SpA is arguably the most intuitive of the three conditions covered in this review. Inflammatory bowel disease (IBD – Crohn’s disease and ulcerative colitis) is recognized as a common extra-articular feature of SpA; approximately 6 to 14 percent of SpA patients develop IBD, and conversely, subclinical gut inflammation is detectable in the majority of ankylosing spondylitis patients even without overt bowel symptoms. The gut and the joint are anatomically and immunologically linked in this disease family in ways that are more direct than in OA or RA.

The microbial picture in SpA includes an enrichment of enteropathogens not typically associated with OA or RA. Studies cited by the authors have found elevated levels of Salmonella, Shigella, and Campylobacter – intestinal pathogens that cause gastroenteritis – in SpA patients. Reactive arthritis itself is by definition triggered by these enteropathogens; it develops in genetically susceptible individuals (particularly those carrying the HLA-B*27 gene) weeks after a gastrointestinal infection, even after the infection itself has cleared. This clinical reality offers the most direct demonstration anywhere in medicine of the gut-joint axis: a gut infection causes a joint disease in a susceptible host.

The HLA-B*27 gene deserves particular attention in SpA biology. Unlike HLA-DRB1 in RA, which operates through T-cell selection of autoreactive clones, HLA-B*27 contributes to disease through three distinct properties: it has unusual peptide-binding selectivity, it tends to misfold inside cells (particularly in the endoplasmic reticulum – the cellular machinery that manufactures proteins), and it forms aberrant heavy-chain homodimers on cell surfaces. Misfolded HLA-B*27 proteins accumulate in the endoplasmic reticulum, triggering cellular stress responses that activate transcription factors such as CHOP and drive the production of pro-inflammatory cytokines including IL-23, interferon-β, and IL-1. The IL-23/IL-17 axis – elevated IL-23 promotes Th17 cell expansion and IL-17 production – is particularly central to SpA pathogenesis and represents one of the most therapeutically successful targets in modern SpA treatment (biologics targeting IL-17 and IL-23 have transformed the management of ankylosing spondylitis and psoriatic arthritis).

The gut microbiome connects to this biology through the Th17/Treg axis. Gut bacteria that normally promote Treg differentiation (maintaining immunological tolerance) are depleted in SpA patients. When the gut environment shifts toward dysbiosis, Th17 cell activity rises unchecked, sustaining the IL-23/IL-17 cascade that drives entheseal and spinal inflammation in ankylosing spondylitis, and peripheral joint and skin inflammation in psoriatic arthritis.

Diagnosing Gut Dysbiosis: Current Tools

The clinical identification of gut dysbiosis – as opposed to laboratory research characterization of microbiome composition – is an area of active development. The authors review two broad approaches.

The most diagnostically useful current approach involves molecular analysis of stool specimens. Traditional culture-based bacteriology can identify only the minority of gut bacteria that grow in standard laboratory conditions; most gut microorganisms are anaerobes that do not survive conventional culture. Molecular approaches – particularly sequencing of the 16S ribosomal RNA gene, which is present in all bacteria but has regions that vary between species – allow researchers and clinicians to characterize the full bacterial community from a stool sample without needing to grow the organisms. The authors describe a specific bacterial marker profiling approach using 54 probes targeting the 16S rRNA gene (V3–V7 regions) covering over 300 bacterial markers; this produces a dysbiosis index score from 1 to 5, where a score of 2 or above is considered indicative of dysbiosis. Taxon-based approaches using the abundance of specific, clinically relevant bacterial species to calculate dysbiosis indices represent a complementary method.

For diagnosing the joint diseases themselves, the authors review the established clinical, serological, and imaging tools – anti-CCP antibodies and rheumatoid factor for RA; HLA-B*27 testing and sacroiliac joint imaging (plain X-ray for established disease, MRI for early or non-radiographic disease) for SpA; and a suite of inflammatory markers including ESR and CRP that reflect systemic inflammatory burden across all arthritis types. Of particular note is the potential future convergence of these diagnostic approaches: if specific dysbiosis patterns reliably precede or predict particular arthritis conditions, microbiome profiling could eventually function as an early warning biomarker – a non-invasive screening tool for individuals at genetic risk, enabling intervention before joint damage begins.

Outcome Measurement Tools: How Researchers Track Disease and Treatment

A substantial portion of the review is devoted to cataloguing the validated outcome measurement instruments used in arthritis research. Understanding these tools is important for interpreting clinical trial results – both conventional drug trials and emerging microbiota-targeted intervention studies. The major instruments reviewed by the authors include the following.

Physical function and disability: The Health Assessment Questionnaire (HAQ) and its modified version (MHAQ) capture patients’ ability to perform activities of daily living across eight domains, scored 0 (no difficulty) to 3 (unable to perform), on a 0–3 scale. The Bath Ankylosing Spondylitis Functional Index (BASFI) is a 10-item self-report instrument specific to ankylosing spondylitis, asking about functional anatomy tasks (bending, reaching, standing) and daily living tasks, scored 0–10 per item. The Arthritis Impact Measurement Scale-2 (AIMS-2) provides a broader multidimensional assessment across physical, symptomatic, role, social, and mood domains through 57 items on 12 scales. The Disabilities of the Arm, Shoulder, and Hand (DASH) questionnaire targets upper limb disability with 30 items converted to a 0–100 scale where higher scores represent greater disability. The Grip Ability Test (GAT) provides an objective timed assessment of hand function in RA patients through three standardized tasks.

Pain: The Visual Analog Scale (VAS) – a 10-centimeter line anchored by “no discomfort” and “worst conceivable pain” – remains one of the most widely used pain assessment tools across arthritis research. The Numeric Rating Scale (NRS) is an 11-point scale (0–10) that is preferred by patients with chronic pain for its simplicity, though it may not capture the full complexity of chronic joint pain as well as multidimensional instruments.

Disease activity composites: The Disease Activity Score in 28 joints (DAS28) – which appeared in the curcumin article earlier in this series – integrates tender joint count, swollen joint count, an inflammatory marker (ESR or CRP), and patient global assessment into a composite score. The Rheumatoid Arthritis Disease Activity Index (RADAI) asks patients directly about pain in 16 joint groups and morning stiffness, producing a 0–10 score. The Ritchie Articular Index (RAI) evaluates tenderness across 52 joints on a 0–3 scale per joint (non-tender to tender-with-withdrawal), with a total range of 0–78.

Fatigue: The Multidimensional Assessment of Fatigue (MAF) – developed specifically for RA in 1991 – captures four dimensions of fatigue: severity, distress, impairment of daily activities, and frequency of change, through 15 items. Its use has since been extended to OA, ankylosing spondylitis, and other rheumatological conditions.

Therapeutic Options Targeting the Gut-Joint Axis

The clinical translation of gut-joint axis research is at an early but rapidly accelerating stage. The authors review the emerging landscape of microbiota-targeted therapeutic strategies alongside existing conventional treatments.

Osteoarthritis: Diet, Probiotics, Exercise, and Fecal Transplantation

Current standard OA treatment is symptom-focused: analgesics (paracetamol), NSAIDs, intra-articular corticosteroid or hyaluronic acid injections, physical therapy, and ultimately joint replacement surgery when conservative measures fail. None of these approaches modify the underlying disease process. Microbiota-targeted strategies offer a potentially disease-modifying dimension.

Probiotics – live beneficial bacterial strains consumed orally – have been tested in OA in multiple randomized, double-blind, placebo-controlled studies and have shown evidence of symptom benefit. Specific genera with supporting evidence include Lactobacillus and Bifidobacterium, which have demonstrated the ability to reduce circulating C-reactive protein in OA patients – suggesting systemic anti-inflammatory effects operating through gut microbiota modulation. The mechanism involves promoting growth of beneficial microorganisms that strengthen the gut barrier, reducing LPS translocation, and shifting the immune microenvironment toward a less pro-inflammatory state.

Diet is a powerful modulator of the gut microbiome. The authors note that increased dietary fiber consumption has been linked in multiple studies to reduced OA-related pain – an association plausibly mediated through the microbiome, because dietary fiber is the primary fuel for the short-chain fatty acid-producing bacteria whose populations contract in dysbiosis. Short-chain fatty acids – particularly butyrate, propionate, and acetate – have direct anti-inflammatory effects: they strengthen intestinal tight junctions, reduce gut permeability, promote Treg differentiation, and suppress pro-inflammatory cytokine production. Conversely, reducing a high-fat diet moderates OA-associated dysbiosis, though the authors note this is unlikely to reverse established structural OA on its own. Vitamin D has also emerged as a microbiome modulator in addition to its established roles in bone and muscle health: evidence reviewed by the authors shows that vitamin D supplementation increases populations of beneficial bacteria such as Ruminococcaceae and Faecalibacterium while reducing Firmicutes overgrowth.

Exercise represents a particularly interesting area of gut-joint axis research. Physical activity maintains gut microbiome diversity and reduces systemic inflammation independent of its direct effects on body composition and joint loading. Notably, the authors report evidence that exercise maintains intestinal structural integrity and lowers systemic inflammation even in the context of a high-fat diet – suggesting that physical activity influences gut microbiota through mechanisms distinct from dietary factors, rather than simply by reducing obesity. This indicates that exercise and dietary modification may work through complementary rather than redundant microbiome pathways.

Fecal microbiota transplantation (FMT) – the transfer of stool from a healthy donor into a recipient’s gastrointestinal tract – has established efficacy in Clostridioides difficile infection (a gut infection resistant to antibiotics) and is under investigation for OA. Animal studies have demonstrated that FMT from OA-affected mice can alter the joint disease phenotype in recipient animals, providing mechanistic proof of concept that the gut microbiome directly influences joint pathology. Human OA trials are in early stages.

Rheumatoid Arthritis: Conventional Treatment and Microbiome Interactions

RA is currently treated with a pyramid of disease-modifying anti-rheumatic drugs (DMARDs). Conventional synthetic DMARDs – led by methotrexate, the global anchor treatment for RA – reduce joint inflammation and slow structural damage. Biological DMARDs target specific inflammatory molecules: TNF-α inhibitors (adalimumab, etanercept, infliximab), IL-6 receptor antagonists (tocilizumab), B-cell depleting agents (rituximab), and co-stimulation blockers (abatacept). Targeted synthetic DMARDs including JAK inhibitors (tofacitinib, baricitinib) represent the newest class.

A particularly intriguing dimension of the gut-joint axis in RA is the bidirectional interaction between these standard treatments and the gut microbiome. Research cited by the authors shows that the gut microbiome composition partially determines the bioavailability and clinical efficacy of methotrexate – meaning that patients with different gut microbial profiles may absorb and respond to the same methotrexate dose differently. Conversely, methotrexate treatment itself partially restores normal gut microbiome composition in RA patients, suggesting that some of its therapeutic benefit may be mediated indirectly through microbiome normalization. This relationship – where a drug affects the microbiome and the microbiome affects the drug – is increasingly recognized as clinically important and is studied under the emerging field of pharmacomicrobiomics.

Probiotic supplementation in RA has shown promising early results. Studies have demonstrated that specific Lactobacillus strains can improve RA disease activity scores and reduce inflammatory markers including CRP. FMT has been used in at least one documented case of refractory RA (RA that failed to respond to multiple conventional treatments) with reported benefit, though clinical trials examining FMT efficacy in RA populations remain scarce and the evidence is anecdotal at this stage.

Spondylarthritis: The IL-23/IL-17 Pathway and the Microbiome

Biological therapies targeting the IL-17 axis (secukinumab, ixekizumab) and the IL-23 pathway (guselkumab, risankizumab) have transformed outcomes in ankylosing spondylitis and psoriatic arthritis over the last decade, providing powerful validation of these pathways as central to SpA pathogenesis. The gut-joint axis offers a biological explanation for why these pathways are so important in this disease family: the gut microbiome, through its effects on Th17 and Treg populations, is a primary upstream driver of IL-17 and IL-23 activity. In SpA, gut dysbiosis may be sustaining the very cytokine cascade that biologics must pharmacologically suppress.

This raises a strategically important future question: could correcting the underlying gut dysbiosis through probiotic, dietary, or FMT interventions reduce the inflammatory drive upstream of the IL-23/IL-17 axis, potentially modifying disease in ways that complement – or even reduce the required intensity of – biological drug treatment? The authors identify this as a priority area for future research, noting that microbiota-targeted therapies could theoretically serve as adjuncts to conventional treatment, reducing disease burden through complementary mechanisms rather than competing with established pharmacological approaches.

The Limitations: What the Evidence Does Not Yet Establish

As a narrative review – rather than a systematic review or meta-analysis – this paper does not include a formal risk-of-bias assessment of the underlying studies it cites, nor does it quantitatively pool data. Its strength lies in synthesizing a rapidly evolving literature across multiple disease areas and research disciplines; its limitation is that it cannot assign precise effect sizes or quality ratings to the evidence it describes.

The most important unresolved question across the entire gut-joint axis literature is causality. While dysbiosis consistently correlates with arthritis in both cross-sectional and longitudinal studies, and while animal models can demonstrate that gut microbiome manipulation alters joint disease phenotype, it has not been definitively established in humans that gut dysbiosis precedes arthritis onset and contributes causally to it – rather than being a consequence of having arthritis (due to the effects of disease, medications like NSAIDs and DMARDs, reduced mobility, and altered diet on the gut environment). The preclinical RA data (showing dysbiosis before joint symptoms appear) provides the strongest evidence available for a causal direction, but fully separating cause from consequence in a complex chronic disease with long pre-symptomatic periods remains methodologically challenging.

The specific bacterial species and metabolites most relevant to each arthritis condition remain incompletely characterized. Different studies using different populations, different microbiome sequencing methods, and different analytical pipelines have produced varying and sometimes conflicting species-level findings. The field has not yet converged on validated dysbiosis signatures that could be used reliably for clinical diagnosis or prognosis, despite the theoretical promise of microbiome profiling as a non-invasive biomarker.

Microbiota-targeted therapies – probiotics, dietary interventions, FMT – are at early stages of clinical validation for most arthritis indications. Probiotic trials in OA and RA have shown positive signals but are generally small, short-term, and heterogeneous in their choice of bacterial strains and doses. FMT trials in arthritis are almost entirely absent. The optimal microbial donor selection, transplantation protocol, and recipient population for FMT in any arthritis condition remain undefined. These are necessary prerequisites for clinical application.

Finally, the review does not address the gut-joint axis in other important arthritis conditions – gout, psoriatic arthritis, lupus, and fibromyalgia among them – reflecting either an absence of sufficient literature or the deliberate scope of this particular review. Readers interested in those conditions should note that gut microbiome research in all of them is active and expanding.

Summary of Key Takeaways

• The gut-joint axis describes a set of biological pathways through which gut microbiota dysbiosis – a harmful imbalance in the intestinal microbial community – drives systemic and local joint inflammation. The three primary mechanisms are bacterial translocation through a leaky gut (with LPS triggering systemic cytokine cascades), molecular mimicry (where bacterial proteins trigger immune responses that cross-react with joint tissue), and disruption of the Treg/Th17 immune balance toward a pro-inflammatory state.
• In osteoarthritis, gut dysbiosis characterized by elevated Actinobacteriota and Proteobacteria alongside reduced Firmicutes contributes to the chronic low-grade joint inflammation now recognized as central to OA pathogenesis – alongside and interacting with the mechanical factors long considered OA’s primary cause.
• In rheumatoid arthritis, gut microbiome changes – including enrichment of Prevotella, Fusobacterium, Porphyromonas, and Bacteroides alongside depletion of protective species – are detectable in preclinical stages before joint symptoms appear, suggesting a causal role in disease initiation rather than mere secondary effect. Bacterial DNA from these genera has been found in RA patients’ serum and synovial fluid.
• In spondylarthritis, gut dysbiosis including enrichment of enteropathogens (Salmonella, Shigella, Campylobacter) connects to joint inflammation through the IL-23/IL-17 axis and the Treg/Th17 imbalance, with the gut-joint relationship being anatomically and immunologically more direct than in other arthritis forms – illustrated most clearly by reactive arthritis, which is by definition a joint disease triggered by a gut infection.
• Stool-based molecular microbiome profiling (16S rRNA gene sequencing with dysbiosis index scoring) offers a non-invasive diagnostic window into gut dysbiosis that may eventually complement standard blood-based and imaging-based arthritis diagnostics – and potentially identify disease-prone individuals before joint symptoms emerge.
• Probiotics (particularly Lactobacillus and Bifidobacterium species) have demonstrated reductions in CRP and symptom improvement in OA and RA in randomized placebo-controlled trials. Dietary fiber and micronutrients including vitamin D modulate the microbiome in directions consistent with reduced joint inflammation. Exercise maintains gut microbiome diversity and gut barrier integrity through pathways independent of its direct anti-obesity effects.
• The gut microbiome interacts bidirectionally with conventional RA drugs: gut microbial composition influences methotrexate bioavailability and clinical response, while methotrexate treatment partially restores normal microbiome composition. This pharmacomicrobiomic relationship has implications for personalizing RA treatment.
• Causality between gut dysbiosis and arthritis onset has not been definitively established in humans; validated diagnostic dysbiosis signatures for arthritis prediction are not yet available; probiotic and FMT trials in arthritis remain early-stage; and the optimal microbiota-targeted intervention strategies are undefined. The gut-joint axis is a scientifically compelling and clinically promising framework, but most therapeutic applications remain investigational.
• The authors call for future research to focus on developing validated microbiota-based screening protocols for predicting arthritis onset in at-risk individuals, and on well-designed clinical trials of microbiota-targeted interventions – including FMT, specific probiotic strains, and dietary protocols – measuring both gut and joint outcomes in OA, RA, and SpA patients.

Longo, Umile Giuseppe, Alberto Lalli, Benedetta Bandini, Roberto de Sire, Silvia Angeletti, Sebastien Lustig, Antonio Ammendolia, Nicolaas Cyrillus Budhiparama, and Alessandro de Sire. “Role of the Gut Microbiota in Osteoarthritis, Rheumatoid Arthritis, and Spondylarthritis: An Update on the Gut–Joint Axis.” International Journal of Molecular Sciences, vol. 25, no. 6, 2024, article 3242. https://doi.org/10.3390/ijms25063242. Full text available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC10970477/.