Gut-ocular axis: An emerging strategy to treat ocular diseases

Abstract

The gut-eye axis is a bidirectional communication pathway linking gut microbiota to ocular health, where a dysbiotic gut can contribute to eye diseases like age related macular degeneration, diabetic retinopathy, uveitis, and dry eyes; even ocular conditions can also influence the gut. This gut - ocular axis offers new therapeutic targets using probiotics, prebiotics, dietary modification, or novel methods like fecal microbiota transplantation to improve eye health. This review also discusses future research prospects in the gut-eye axis, while addressing a research gap in the existing literature.

Key Words: Gue-eye axis; Dysbiosis; Fecal microbiota transplantation; Leaky gut; Diabetic retinopathy

Core Tip: The gut-eye axis highlights a critical relationship between gut microbiota and eye. Gut dysbiosis can promote inflammatory and immune-mediated eye diseases. Microbiome-targeted interventions such as probiotics, prebiotics, dietary modification, and fecal microbiota transplantation represent promising therapeutic strategies for maintaining and improving eye health.

INTRODUCTION

The human gut microbiome is a complex, diverse and dynamic ecosystem composed of microorganisms, that collectively play a pivotal role in maintaining homeostasis in our body[1-3]. In addition to the role of the microbiome in digestion, newer aspects of the potential role of the gut microbiome, such as regulation of immune system maturation, epithelial integrity and cellular signalling, are being explored[4,5]. Advances in next-generation sequencing and metabolomic studies have further unearthed the influence of the microbiome on both local and systemic health[2,6].

Intestinal dysbiosis is an alteration in the gut microbiota composition and diversity[7]. Many hypotheses have been generated to associate this dysbiosis with systemic disorders like non-alcoholic fatty liver disease, autoimmune disorders, neurodegenerative diseases, cardiovascular conditions, and various malignancies[3,8,9]. Apart from gastrointestinal tract, gut-derived immune mediators, microbial metabolites, and neuroendocrine signals also enhance the communication between the gut and distant organs[5,6,9].

The concept of cross-talk via microbial signalling is well established through the study of the gut-brain, gut-lung, and gut-liver axes[10,11]. Recently, the gut-ocular axis has also emerged as a novel and rapidly evolving field of research. There is now credible evidence that the gut microbiome can influence ocular health through neuroinflammatory and immunological mechanisms[7,10].

The eye is considered an immune-privileged organ due to the protection of blood-retinal barrier, blood-aqueous barrier, and regulatory immune environments such as anterior chamber-associated immune deviation[12]. However, emerging evidence suggests that the eye is not fully isolated from systemic influences. Circulating cytokines, migrating immune cells, oxidative stress, and metabolic disturbances can all reach and affect ocular tissues[8,13]. Intestinal dysbiosis has been found to disrupt ocular immune homeostasis, and hence contribute in pathogenesis of various ocular disorders. Hence, the knowledge of the gut-ocular axis has given rise to a paradigm shift, redefining ocular disorders as conditions influenced by systemic disturbances rather than a localized disease process[10].

Previously, several reviews have addressed the association between the gut microbiome and ocular health[14]. Still, most have focused on individual diseases or pathways and have not provided a unified translational perspective[14,15]. This review attempts to fill this gap by providing an extensive cross-disease assessment of the gut-ocular axis. It integrates immunological, metabolic, neuroimmune and barrier mechanisms into a single framework which also includes the latest multi-omics data and clinical trial evidence. Additionally, this review differs from previous published literature by critically evaluating the level of evidence for each ocular disease, identifying disease-specific microbial signatures, and proposing a structured roadmap for translational and clinical research in this rapidly evolving field.

LITERATURE SEARCH METHODOLOGY

A thorough literature search was conducted to ensure a structured and transparent selection of the evidence. The following electronic databases were searched: PubMed/MEDLINE, Scopus, Web of Science, and the Cochrane Library. The search terms included: “gut microbiome”, “gut microbiota”, “intestinal dysbiosis”, “gut-ocular axis”, “gut-eye axis”, “ocular surface”, “uveitis”, “glaucoma”, “diabetic retinopathy”, “age-related macular degeneration”, “dry eye disease”, “probiotics”, “fecal microbiota transplantation”, “short-chain fatty acids”, and “microbiome-targeted therapy”. These terms were combined with Boolean operators (AND, OR) to increase the reach. The publication period was restricted to articles published between January 2007 and June 2025. Peer-reviewed original research articles, systematic reviews, meta-analyses, and narrative reviews published in the English language were included. Letters, editorials, conference abstracts without full-text availability, and non-peer-reviewed preprints were excluded. Reference lists of selected articles were hand-searched for additional relevant publications.

MECHANISMS OF THE GUT-OCULAR AXIS

The gut-ocular axis is mediated by a complex network involving epithelial integrity, oxidative stress, and neuroimmune interactions[11]. These mechanisms explain how intestinal dysbiosis can influence ocular tissues. Figure 1 depicts the complex interaction between host ophthalmic immunity and gut microbiota.

Figure 1 The gut-ocular axis. This diagram depicts the communication between the gut microbiota and the ocular tissue, highlighting the roles of immune cells (Th17, Treg), microbial metabolites (short-chain fatty acid, lipopolysaccharides), and the nerves (vagus nerve). SCFAs: Short-chain fatty acids.

Immune modulation

Immune modulation links the gut microbiota to ocular health. The gut microbiome plays a key role in immune modulation by influencing antigen presentation, dendritic cell maturation, and T-cell differentiation[1,4]. Under physiological conditions, commensal microorganisms promote the development of regulatory T cells, pivotal for maintaining immune tolerance, while balancing pro-inflammatory T-helper subsets such as Th1 and Th17 cells[4,5].

When there is intestinal dysbiosis, this equilibrium breaks down. Reduced Treg activity and enhanced Th17 responses augment pro-inflammatory signalling[4,7]. Th17 cells produce interleukin-17, a cytokine strongly associated with autoimmune inflammation. These immune alterations do not remain confined to the gut. Rather, these immune cells acquire a pro-inflammatory phenotype and migrate to distant organs including ocular structures[7-9].

This is relevant to ocular inflammatory diseases such as uveitis, in which auto-reactive T cells originating in the gut have been shown to infiltrate ocular tissues and trigger inflammation. Molecular mimicry between antigens and ocular self-antigens may increase autoimmune responses[7]. Despite the presence of immune-regulatory mechanisms in the eye, systemic immune dysregulation driven by gut microbiota can overwhelm local tolerance, leading to chronic ocular inflammation[10].

Intestinal barrier dysfunction and systemic inflammation

The intestinal epithelial barrier integrity is a key part of systemic immune homeostasis[12,13]. Tight junction proteins prevent the translocation of microbes into the bloodstream[13]. Dysbiosis disrupts these structures, resulting in increased intestinal permeability, commonly referred to as “leaky gut”[12,13].

Increased permeability allows bacterial products such as lipopolysaccharides to enter systemic circulation. Circulating endotoxins activate innate immune pathways through toll-like receptor signalling, triggering the release of pro-inflammatory cytokines and sustaining chronic low-grade inflammation. Persistent systemic inflammatory signalling can impair vascular, neural, and epithelial tissues, including those within the eye (Figure 2).

Figure 2 Pathophysiology of intestinal dysbiosis-induced ocular inflammation. This flowchart depicts the pathogenic pathways starting from intestinal dysbiosis, systemic inflammation, and amplifying the chronic inflammation of the eye. LPS: Lipopolysaccharide.

This mechanism provides a direct biological link between gut dysfunction and ocular disease. Retinal vascular injury, neuroinflammation, and breakdown of ocular barriers have all been associated with systemic inflammatory states[16,17]. Intestinal barrier dysfunction is therefore particularly relevant in metabolic and age-related ocular diseases, such as diabetic retinopathy and age-related macular degeneration[17,18].

MICROBIAL METABOLITES, OXIDATIVE STRESS, AND METABOLIC SIGNALLING

Microbial metabolites represent a crucial communication pathway between the gut and distant organs. Among the most extensively studied are short-chain fatty acids (SCFAs), including acetate, propionate, and butyrate, which are produced through microbial fermentation of dietary fibres. SCFAs help to reduce inflammation, strengthen barrier integrity, and do immunomodulation[11].

Intestinal dysbiosis is associated with reduced SCFA production and altered metabolomic profiles. This leads to immune dysregulation and increased oxidative stress[11,17] Oxidative stress is important for retinal degeneration, vascular injury and neuronal injury[17,19].

Microbial metabolism of bile acids and tryptophan generates bioactive compounds. These metabolites influence immune activity, and oxidative pathways[18]. These metabolite-mediated mechanisms have been found to be associated with glaucoma, uveitis, diabetic retinopathy, and age-related macular degeneration (AMD)[20,21]. Together, these findings highlight the importance of metabolic signalling in the gut-ocular axis[19,22].

Beyond SCFAs, several other microbial metabolites are now recognized as important mediators of gut-ocular crosstalk. Tryptophan metabolites, particularly indole and its derivatives such as indole-3-aldehyde and indole-3-propionic acid, act on the aryl hydrocarbon receptor (AhR) in immune cells and epithelial surfaces[23]. AhR activation promotes intestinal barrier integrity and modulates dendritic cell function, both of which have downstream consequences for ocular immune homeostasis. Reduced tryptophan catabolism by commensal bacteria has been observed in patients with inflammatory bowel disease and uveitis, supporting a shared metabolic pathway.

Bile acid metabolism also warrants attention. Primary bile acids synthesized in the liver undergo biotransformation by gut bacteria into secondary bile acids, including deoxycholic acid and lithocholic acid. These secondary bile acids activate the farnesoid X receptor and the G-protein-coupled bile acid receptor, both of which regulate inflammatory signalling cascades. Alterations in bile acid profiles have been documented in diabetic patients, and their potential contribution to retinal vascular dysfunction is an area of growing interest[24].

Trimethylamine N-oxide (TMAO), generated from the microbial metabolism of dietary choline, betaine, and L-carnitine, is another metabolite of clinical relevance[25]. Elevated circulating TMAO concentrations have been linked to endothelial dysfunction, systemic vascular inflammation, and atherosclerosis. Given that diabetic retinopathy and age-related macular degeneration share common vascular risk factors, TMAO may serve as a link between dietary intake, gut microbial activity, and retinal vascular pathology.

Neuroimmune and neurodegenerative pathways

The gut microbiome also communicates with the nervous system through neuroimmune interactions. Microbial signals influence microglial activation, the resident immune cells of the central nervous system, including the retina and optic nerve[20,26] Chronic microglial activation drives neuroinflammation and neuronal damage, processes that are central to neurodegenerative ocular diseases[26-29].

The gut microbiome interacts with the brain via systemic signalling molecules and neural pathways such as the vagus nerve, forming part of the broader gut-brain axis[10,20]. Dysregulated gut-brain-eye signalling may therefore contribute to retinal ganglion cell loss, optic nerve degeneration, and impaired neuronal survival. These neuroimmune mechanisms are particularly relevant in glaucoma, where immune-mediated neurodegeneration plays a crucial role independent of intraocular pressure[21,22].

Integrated mechanistic crosstalk

Although immune, metabolic, and neural pathways have been described separately for clarity, they do not operate in isolation. In clinical disease states, these pathways converge and amplify one another. For instance, intestinal dysbiosis simultaneously reduces SCFA production (metabolic pathway), triggers Th17 cell expansion (immune pathway), and activates vagal afferents that alter microglial polarization in the retina (neural pathway). The resulting oxidative stress further damages the intestinal epithelium, perpetuating a vicious cycle of barrier dysfunction, systemic inflammation, and ocular tissue injury. This integrated perspective is essential for understanding why single-target therapies have shown limited efficacy in chronic ocular diseases with a gut-immune component. Future therapeutic strategies should therefore aim to address multiple nodes of this interconnected network rather than targeting isolated pathways.

IMPACT OF THE GUT MICROBIOME ON SPECIFIC OCULAR DISEASES

Cornea and ocular surface disorders

The ocular surface represents a highly sensitive interface between the immune system and the external environment. It is susceptible to the pro-inflammatory effects of systemic cytokines and microbial pathogens. Experimental studies have demonstrated that intestinal dysbiosis affects corneal epithelial healing, nerve regeneration[19,20,26,30]. Table 1 summarize various gut microbiota alterations, their impact in different eye diseases and potential therapeutic targets.

Table 1 Gut microbiome alterations, mechanisms, and therapeutic targets in ocular diseases.

Ocular disease	Gut microbiome alteration	Proposed mechanism	Major evidence	Potential therapy
Dry eye disease	↓ Bifidobacterium, ↓ Faecalibacterium prausnitzii; ↑ Enterobacteriaceae, ↑ Prevotella	Reduced SCFA production → systemic inflammation → lacrimal gland dysfunction → tear film instability	Cross-sectional human cohorts[20]; murine germ-free models[10]	Probiotics (Lactobacillus, Bifidobacterium); dietary fibre; pilot FMT
Uveitis	↓ Firmicutes (Faecalibacterium prausnitzii, Roseburia); ↓ Akkermansia muciniphila; ↑ Bacteroidetes	Th17 expansion → autoreactive T-cell migration to uveal tissue; molecular mimicry	EAU murine models; case-control human studies (Behçet, HLA-B27)[31]	Probiotics; SCFA supplementation; FMT (case reports)
Glaucoma	↑ Enterobacteriaceae; ↓ Lactobacillus, ↓ Bifidobacterium	Commensal-induced T-cell priming → microglial activation → retinal ganglion cell loss	Murine commensal T-cell models; preliminary human cross-sectional data[10,21]	Probiotics; anti-inflammatory dietary patterns
Diabetic Retinopathy	↓ Roseburia, ↓ Faecalibacterium prausnitzii, ↓ Akkermansia muciniphila; ↑ Escherichia, ↑ Shigella	Metabolic endotoxaemia → retinal endothelial activation → BRB breakdown → pericyte loss	Rodent DM models[28]; cross-sectional human microbiome profiling[12]	Dietary modification; prebiotics; glycaemic control with microbiome monitoring
Age-related macular degeneration	↑ Prevotella, ↑ Ruminococcus; ↓ Bacteroides, ↓ Bifidobacterium	High-fat diet → pro-inflammatory microbiota → complement activation → choroidal neovascularization	Murine high-fat diet models[25,26]; case-control human gut microbiome studies[14,22]	Mediterranean diet; omega-3 supplementation; FMT (experimental)

SCFA: Short chain fatty acid; FMT: Fecal microbiota transplantation; DM: Diabetes mellitus; HLA: Human leukocyte antigen; BRB: Blood retina barrier.

Intestinal dysbiosis triggered by a high-fat diet has been shown to impair corneal wound healing. Similarly, antibiotic intake, by reducing intestinal microbiota diversity, alters macrophage activity.

Systemic immune dysregulation driven by intestinal dysbiosis may alter the inflammatory milieu, rendering the ocular surface susceptible to pathogens. These findings point to the contribution of intestinal dysbiosis towards both the development and maintenance of corneal disease[26].

Dry eye disease

Dry eye disease is characterized by the instability of the tear film, inflammation, and neurosensory abnormalities. Latest research suggests that it is a systemic inflammatory disease entity, influenced by gut microbiota rather than a localised disease process[19,20].

Patients with dry eye disease often exhibit reduced intestinal microbial diversity. Dysbiosis activates the systemic immune cascade and oxidative stress, which trigger lacrimal gland function and tear secretion. The gut-eye-lacrimal gland axis links the gut microbiota to tear film abnormalities and ocular surface inflammation[19,20].

Studies have reported that patients with dry eye disease, particularly those with concomitant Sjögren syndrome, show a significant reduction in Bifidobacterium and Faecalibacterium prausnitzii, both of which are key producers of butyrate and other anti-inflammatory SCFAs[27]. There is also an enrichment of Enterobacteriaceae and Prevotella species, which are associated with a pro-inflammatory gut environment. The resulting depletion of SCFA-producing commensals may weaken the intestinal barrier and promote systemic inflammatory mediators that reach the lacrimal gland and ocular surface.

Therapies targeting the gut microbiome, such as probiotics and fecal microbiota transplantation, have shown promise in reducing inflammation and improving symptoms in selected patients. These approaches highlight the potential of microbiome modulation as a novel treatment avenue for dry eye disease[19,20].

Uveitis

Uveitis is a prototype example of the gut-ocular axis disorder. It is frequently associated with systemic immune dysregulation[7,9].

Animal model studies have clearly shown that gut microbiota influence both the development of uveitis and its clinical severity. Dysbiosis activates auto-reactive T cells, which migrate from the intestine to the eye and initiate local inflammation. Microbial metabolites such as SCFAs and bile acids protect against ongoing inflammation by modulating immune responses[7,11].

Patients with autoimmune uveitis, including those with Behçet disease and HLA-B27-associated anterior uveitis, demonstrate a characteristic reduction of Firmicutes (especially Faecalibacterium prausnitzii and Roseburia) and an over-representation of Bacteroidetes[30,31]. There is also a decrease in Akkermansia muciniphila, a mucin-degrading bacterium that plays an important role in mucosal barrier integrity. These alterations shift the Firmicutes-to-Bacteroidetes ratio, favouring a pro-inflammatory intestinal milieu that promotes Th17-mediated ocular inflammation.

These findings raise hope that therapies that alter intestinal dysbiosis can add to the therapeutic armamentarium of refractory or chronic uveitis.

Glaucoma

Glaucoma, a major cause of irreversible blindness globally, is increasingly being recognized as a neuroinflammatory disorder. Apart from the raised intraocular pressure, neuroinflammation also contributes to retinal ganglion cell loss and optic nerve damage.

Emerging evidence suggests that intestinal dysbiosis induces systemic immune activation and microglial inflammation, which may accelerate neurodegeneration in glaucoma. Gut-derived immune cells and inflammatory mediators influence optic nerve health[21,22].

Preliminary studies in both murine models and human cohorts have identified an increased abundance of Enterobacteriaceae and a decreased representation of Lactobacillus and Bifidobacterium in glaucoma patients[29]. Commensal microflora-induced T-cell responses have been shown to mediate progressive neurodegeneration in the retina, with heat shock protein-specific T cells migrating from the gut to the optic nerve head. This suggests that the gut may act as a priming site for neuro-destructive immune responses in glaucoma, independent of intraocular pressure elevation.

Interventions targeting intestinal dysbiosis might help in modulating disease progression alongside traditional pressure-lowering strategies[21].

Diabetic retinopathy

Diabetic retinopathy is a microvascular complication of diabetes mellitus, characterized by retinal inflammation, vascular permeability, and neurodegeneration. Intestinal dysbiosis in these patients has been associated with increased intestinal permeability and systemic inflammation.

In patients with type 2 diabetes and retinopathy, gut microbiome profiling has revealed a depletion of Roseburia, Faecalibacterium, and Akkermansia muciniphila, alongside an enrichment of Escherichia, Shigella, and other opportunistic Enterobacteriaceae[27-31]. This microbial shift is accompanied by elevated circulating lipopolysaccharide concentrations and a disrupted Firmicutes-to-Bacteroidetes ratio. The resulting metabolic endotoxaemia promotes retinal endothelial cell activation, pericyte loss, and breakdown of the blood-retinal barrier, which are hallmarks of diabetic retinopathy progression.

These events promote retinal vascular dysfunction, oxidative stress, and chronic inflammation, exacerbating disease progression. Understanding the role of the gut microbiome in diabetic retinopathy may lead to newer preventive measures, particularly through dietary interventions and therapies targeting dysbiosis[15,16].

Age-related macular degeneration

AMD is a multifactorial disease influenced by environmental exposure, diet, and metabolic factors in genetically susceptible individuals. Latest studies suggest that intestinal dysbiosis contributes to AMD pathogenesis[24,25].

Intestinal dysbiosis caused by diet, affect retinal inflammation and oxidative stress. Specific microbial profiles and the development of neovascular AMD seem to be associated, suggesting a link between systemic microbial imbalance and retinal degeneration[25].

Patients with neovascular AMD have been found to harbour increased levels of Prevotella and Ruminococcus and decreased levels of Bacteroides and Bifidobacterium compared to age-matched controls[24,25]. A high-fat, high-glycaemic diet has been shown in murine models to shift the gut microbiota towards a pro-inflammatory composition, resulting in elevated choroidal neovascularization. These observations suggest that dietary-induced microbial alterations may predispose the ageing retina to complement activation, drusen formation, and photoreceptor degeneration through chronic low-grade systemic inflammation.

Dietary therapies or fecal microbiota transplantation may alter the disease risk and progression, opening newer avenues for treatment[25].

HIERARCHY OF EVIDENCE FOR GUT-OCULAR AXIS ASSOCIATIONS

The evidence supporting the gut-ocular axis varies considerably across disease entities. Most mechanistic insights originate from animal studies, while human clinical data remain limited to observational cohorts and a small number of interventional studies. Table 2 summarizes the current level of evidence for each major ocular condition discussed in this review.

Table 2 Current level of evidence for each major ocular condition.

Ocular disease	Animal studies	Human observational studies	Clinical trials/meta-analyses
Dry eye disease	Multiple murine models (germ-free, antibiotic-treated)[10]	Cross-sectional and cohort studies (Sjögren and non-Sjögren)[20]	Limited; pilot probiotic trials
Uveitis	Extensive (EAU models, gnotobiotic mice)[8,9]	Case-control studies (Behçet, HLA-B27)[31]	No published RCTs; case series with FMT
Glaucoma	Murine models (commensal T-cell priming)	Preliminary cross-sectional studies[10,21]	None published to date
Diabetic retinopathy	Rodent models of type 1 and type 2 diabetes[28]	Cross-sectional gut microbiome profiling[12]	No dedicated ocular endpoint trials
Age-related macular degeneration	High-fat diet murine models[25,26]	Case-control gut microbiome studies[14,22]	No published RCTs

HLA: Human leukocyte antigen; RCT: Randomized controlled trial; FMT: Fecal microbiota transplantation.

CLINICAL TRIAL EVIDENCE FOR MICROBIOME-TARGETED OCULAR THERAPIES

The clinical translation of gut-ocular axis research remains at an early stage, and a balanced appraisal of the available interventional data is essential. To date, clinical trial evidence in this field is sparse and largely confined to pilot or small-scale studies. Probiotics: A handful of small, open-label studies have evaluated probiotic supplementation in patients with dry eye disease. A pilot randomized study involving 40 patients with Sjögren syndrome-associated dry eye reported that an 8-week course of a multi-strain probiotic (containing Lactobacillus and Bifidobacterium species) led to modest improvements in Schirmer test scores and tear breakup time compared to placebo[32]. However, the sample size was limited, follow-up was short, and the study lacked microbiome sequencing to confirm microbial shifts.

Fecal microbiota transplantation: Fecal microbiota transplantation (FMT) has been explored in isolated case reports for refractory autoimmune uveitis, with anecdotal improvement in disease activity following donor stool engraftment[33]. However, no controlled trials have been conducted in ophthalmological indications. Safety concerns, including the risk of transmitting drug-resistant organisms and the absence of standardized donor screening protocols, remain significant barriers to wider adoption.

Dietary interventions: Observational data suggest that adherence to a Mediterranean diet pattern, rich in fibre, omega-3 fatty acids, and polyphenols, is associated with lower rates of advanced AMD[34]. While these associations are plausible and consistent with microbiome-mediated anti-inflammatory effects, they do not establish causality, and no large-scale dietary intervention trial with microbiome monitoring and ophthalmic outcomes has been completed to date.

Taken together, the available clinical trial evidence supports the biological plausibility of microbiome-targeted approaches in ophthalmology but does not yet provide sufficient data to guide clinical practice. Adequately powered, multi-centre, randomized controlled trials with standardized ophthalmic endpoints, serial microbiome profiling, and long-term safety monitoring are urgently needed.

FUTURE DIRECTIONS AND TRANSLATIONAL IMPLICATIONS OF THE GUT-OCULAR AXIS ON OCULAR DISEASES

While emerging evidence links gut dysbiosis to a broad spectrum of ophthalmological disorders, clinical studies remain limited[35,36]. The need of the hour is to conduct causality studies, clinical applicability, and precision-based treatment strategies to revolutionize ophthalmic practice[36].

Establishing causality and disease-specific microbial signatures

Future research should distinguish causal microbial drivers of inflammation from secondary alterations associated with ophthalmological disorders[34,35]. In-depth translational studies are required to determine whether gut dysbiosis actively initiates or accelerates the disease process[36,37]. Identifying specific microbial signatures linked to various ophthalmological disorders such as uveitis, glaucoma, dry eye disease, diabetic retinopathy, and age-related macular degeneration may enable the development of novel microbiome-based biomarkers for early diagnosis, disease stratification, and prognostic assessment[38-40].

Integration of multi-omics and systems biology approaches

Advanced multi-omics technologies—including metagenomics, metabolomics, transcriptomics, and proteomics—offer unprecedented opportunities to characterize functional microbial outputs rather than microbial composition alone[41,42]. Integrating these datasets will help elucidate how microbial metabolites, immune mediators, and neuroinflammatory pathways interact across the gut-brain-eye axis, while systems biology approaches may uncover molecular targets linking intestinal dysbiosis to ocular inflammation, vascular dysfunction, and neurodegeneration[41].

In this regard, metatranscriptomics offers the advantage of capturing active gene expression in the gut microbiome, as opposed to metagenomics, which provides only a catalogue of genetic potential[42]. Metabolomics can quantify circulating SCFAs, bile acids, and tryptophan derivatives in serum samples, enabling direct correlation with ophthalmic clinical parameters. Single-cell RNA sequencing of peripheral blood mononuclear cells and retinal tissue can further delineate how gut-primed immune cells undergo phenotypic changes upon reaching ocular structures. Incorporating these technologies into prospective clinical studies will bridge the gap between descriptive associations and functional mechanistic understanding.

Microbiome-targeted therapeutics

Microbiome modulation is poised to emerge as an important adjunct or alternative to conventional ophthalmic therapies (Figure 3). Precision probiotics, prebiotics, postbiotics, dietary interventions, and FMT warrant rigorous evaluation in controlled clinical trials[41]. Defining optimal timing, dosage, safety profiles, and patient-specific indications will be essential for clinical success, particularly in chronic, refractory, or immune-mediated ocular diseases where long-term immunosuppression carries significant risks.

Figure 3 Therapeutic targets and specific ocular disease implications. This diagram depicts two potential therapeutic targets- modulation strategies: How prebiotics, probiotics, diet, and fecal microbiota transplantation can potentially restore microbiota diversity. Targeted ocular diseases: Linking the gut-ocular axis to specific conditions like dry eye, uveitis, glaucoma, and age-related macular degeneration. FMT: Fecal microbiota transplantation; AMD: Age-related macular degeneration; SCFAs: Short-chain fatty acids.

Personalized and preventive ophthalmology

Considerable inter-individual variability in gut microbial composition underscores the need for personalized medicine approaches[36,39]. Incorporating microbiome profiling into routine clinical assessment may allow clinicians to identify individuals at elevated risk for inflammatory or neurodegenerative ocular diseases before irreversible tissue damage occurs. Microbiome-targeted measures can help delay disease onset or slow progression, especially useful for metabolic and age-related disorders such as diabetic retinopathy and age-related macular degeneration.

Neuroimmune and systemic perspectives

The complex interplay between the gut-brain-eye axis can explain how neuroimmune mechanisms lead to retinal and optic nerve degeneration. Unravelling the mechanisms by which gut-derived immune cells and microbial metabolites affect microglial activation, neuronal survival, and synaptic integrity can identify novel therapeutic targets for neurodegenerative ocular diseases such as glaucoma.

Interdisciplinary collaboration

To bring gut-eye axis innovation from bench to bedside demands multi-disciplinary collaboration between ophthalmologists, immunologists, microbiologists, neurologists, and basic scientists. Multicentric clinical trials, cutting-edge microbiome sampling techniques, and engaging regulatory and ethical frameworks will be of utmost importance to revolutionize the clinical practice.

Integrated outlook

The gut-eye axis reimagines ophthalmological disorders as systemic conditions rather than a local pathology. The integration of the latest scientific innovations with clinical acumen can potentially transform diagnostic as well as therapeutic strategies, paving way for precision medicine in patients with ocular disease.

LIMITATIONS OF THE CURRENT EVIDENCE

Several limitations must be acknowledged when interpreting the evidence presented in this review. First, the majority of mechanistic studies have been conducted in murine models, and the degree to which these findings can be extrapolated to human ocular disease remains uncertain. Germ-free and gnotobiotic mouse models, while powerful for establishing causal relationships, do not replicate the complexity of the human gut microbiome and its interactions with host genetics, diet, medications, and environmental exposures.

Second, there is considerable heterogeneity in microbiome study methodologies across published reports. Differences in DNA extraction techniques, 16S rRNA gene sequencing vs whole-genome shotgun metagenomics, bioinformatic pipelines, and reference databases make direct comparisons between studies difficult. This heterogeneity limits the ability to identify consistent disease-specific microbial signatures.

Third, most human studies are cross-sectional or retrospective in design, making it impossible to determine whether observed microbial alterations are causative or merely a consequence of the ocular disease or its treatment. The confounding effects of concurrent antibiotic use, topical and systemic anti-inflammatory medications, dietary habits, age, geography, and ethnicity are rarely controlled for adequately.

Fourth, the influence of diet as a confounding variable is particularly challenging. Dietary patterns are the strongest modifiable determinant of gut microbial composition, yet dietary assessment in most published studies is either absent or based on brief food-frequency questionnaires with inherent recall bias.

Finally, the distinction between causality and association remains the most important unresolved question in this field. Until large-scale, longitudinal, prospective cohort studies with serial microbiome sampling and well-defined ophthalmic endpoints are conducted, the gut-ocular axis will remain a compelling hypothesis rather than a confirmed therapeutic target.

CONCLUSION

The gut-ocular axis is a potential therapeutic target for understanding and identifying novel treatments for various ophthalmological disorders such as dry eye disease, uveitis, glaucoma, diabetic retinopathy, and age-related macular degeneration. This is an evolving field of research; nonetheless, it offers deeper insights for identifying disease-specific microbial signatures and developing newer therapeutic strategies[29]. Finally, the integration of microbiome-focused approaches into ophthalmology may enable personalized, systemic treatment strategies that address the underlying drivers of ocular pathology and improve outcomes across diverse ocular diseases.