Obesity, characterized as a chronic, multifactorial disease, has escalated into a worldwide health crisis, impacting around 641 million individuals globally (NCD-RisC, 2016). It is widely recognized for increasing the risk of numerous health issues including metabolic syndrome, cardiovascular diseases, type 2 diabetes, and hypertension, posing a significant public health challenge (De Andrade Mesquita et al., 2023). Despite the scarcity of research on obesity’s potential effects on eye diseases, it’s crucial to acknowledge its potential impact in this area as well.

Severe obesity may induce irreversible blindness by promoting the occurrence and progression of diseases such as age-related macular degeneration (AMD) and glaucoma. Obesity, particularly when induced early in life, can lead to long-term pro-inflammatory changes in monocytes (Hata et al., 2023). This inflammation exacerbates pathological conditions such as retinal neovascularization and neuron degeneration, closely associated with vision loss and AMD-related blindness (Hata et al., 2023). Furthermore, obesity-related increases in orbital fat, along with decreased choroidal perfusion and ocular blood flow, might lead to higher intraocular pressure, fostering the development and progression of glaucoma (Pezzino et al., 2023). In addition to these factors, blood lipid abnormalities may cause dysfunction of the meibomian glands by disrupting the rheological properties of the eyelid fat or blocking the meibomian gland (Osae et al., 2019). The systemic chronic inflammation, elevated leptin levels, and oxidative stress associated with obesity are also potential contributors to various eye diseases (Cheung and Wong, 2007; Hata et al., 2023). Moreover, Hata et al. discovered that obesity’s effects on the eyes might be lasting, irreparable even with subsequent weight loss (Hata et al., 2023).

The relationship between the gut microbiota and obesity is unmistakable. Regarding obesity-related eye diseases, studies have discovered that the gut microbiota may contribute by inducing increased intestinal permeability and lipopolysaccharide translocation in HFD-fed mice, leading to laser-induced choroidal inflammation and neovascularization (Andriessen et al., 2016). Interestingly, treatment with the lipid-lowering drug fenofibrate in HFD-fed mice has been observed to reduce retinal inflammation and increase the abundance of gut microbiota that produce short-chain fatty acids (SCFAs), offering potential therapeutic insights (Wang et al., 2022). However, with the Human Microbiome Project dramatically broadening the scope of microbiome research to include various ecological niches beyond traditional domains like the gut and oral cavity, the pivotal roles of localized microbiomes, including those on the ocular surface, within their specific environments are garnering more focus (iHMP Research Network Consortium, 2019).

Current research underscores the essential role of the ocular surface microbiome in maintaining ocular homeostasis (Gomes JÁ et al., 2020; Deng et al., 2021; Herzog et al., 2023; Zhao et al., 2023). It achieves this through key mechanisms such as selective immune tolerance (Ueta and Kinoshita, 2010) and the resistance to pathogenic colonization (St Leger et al., 2017), all while being modulated by both local ocular conditions and systemic health factors (Gomes JÁ et al., 2020). Multiple studies have shown that epithelial cells of the cornea and conjunctiva can selectively produce pro-inflammatory factors to respond to specific pathogenic bacteria on the ocular surface without causing inflammation to the commensal bacteria (Cavuoto et al., 2019). Changes in the ocular surface microbiota are closely related to AMD, glaucoma, dry eye syndrome, meibomian gland dysfunction, conjunctivitis, keratitis, and diabetes (Li et al., 2019; Gomes JÁ et al., 2020; Deng et al., 2021).

Similar to the alterations observed in the gut microbiota, obesity-associated chronic low-grade inflammation may also influence the ocular surface microbiota. This can potentially compromise the eye’s protective barrier, thus affecting ocular health through mechanisms that extend beyond well-known systemic pathways. Despite the recognized importance of the ocular surface microbiota in maintaining eye health, the specific changes it undergoes in patients with obesity have yet to be fully investigated. Considering the ocular surface’s accessibility and the pivotal role its microbiome plays in preserving ocular equilibrium, our study seeks to delve into these alterations. We aim to provide new perspectives in ocular surface microbiota research, contributing to the development of strategies for managing and preventing obesity-related eye diseases.

Previous studies employing both traditional methods and 16S rRNA sequencing, have highlighted the presence of a diverse and abundant microbial community on the human ocular surface (Dong et al., 2011; Zegans and Van Gelder, 2014; Zhou et al., 2014; Doan et al., 2016; Shin et al., 2016; Ozkan et al., 2017). These advancements have facilitated research into the ocular surface microbiota in diseases such as dry eye, meibomian gland dysfunction, keratitis, and diabetes, utilizing 16S rRNA technology (Delbeke et al., 2021). However, the composition of the ocular surface microbiota in the context of obesity remains an uncharted area. Additionally, recent studies have raised the possibility that the gut microbiota might contribute to obesity-induced choroidal pathological angiogenesis via the gut-eye axis (Andriessen et al., 2016). The role of the ocular surface microbiota in the onset and progression of obesity-related eye diseases, therefore, warrants further investigation.

In this study, we utilized 16S rRNA sequencing technology to explore the composition of the ocular surface microbiota in patients with obesity. Our analysis revealed a trend towards increased alpha diversity in the obese group compared to healthy group. Furthermore, we noted significant differences in beta diversity, particularly in the distribution of species abundance. The ocular surface microbiota predominantly consisted of the phyla Proteobacteria, Actinobacteria, Firmicutes, and Bacteroidetes. Pseudomonas, Rhodococcus, Aliidiomarina, Corynebacterium, Delftia, and Acinetobacter identified as core genera. Additionally, we found that Delftia and Cutibacterium were more prevalent in the microbiota of patients with obesity. Furthermore, our functional prediction analysis indicated an enrichment in pathways related to CYP450 metabolism, lipid metabolism, and NOD-like receptor signaling in the patients with obesity, alongside a decrease in the ECM receptor pathway.

Delbeke et al. reviewed 76 studies, revealing that the phylum-level ocular microbiome in healthy individuals predominantly comprises Proteobacteria, Actinobacteria, Firmicutes, and Bacteroidetes, with Proteobacteria and Actinobacteria being the most prevalent (Delbeke et al., 2021). At the genus level, the core microbiome typically includes Staphylococcus, Corynebacterium, Propionibacterium (Cutibacterium), Pseudomonas, Acinetobacter, and Streptococcus. Our research found a similar phylum-level composition in both obese and healthy groups, suggesting a degree of stability at this taxonomic level. At the genus level, we also detected the presence of the previously mentioned microbial communities, with Corynebacterium, Pseudomonas, and Acinetobacter remaining as the core microbial genera in both of our study groups. Previous research has documented the presence of Corynebacterium, Pseudomonas, and Acinetobacter across various global populations, albeit with varying abundances (Ozkan and Willcox, 2019; Delbeke et al., 2021). Corynebacterium mastitidis, within the genus Corynebacterium, has been closely linked to the maintenance of ocular immune homeostasis (St Leger et al., 2017). Moreover, Pseudomonas and Acinetobacter have been associated with the development of conditions such as Stevens-Johnson syndrome, type 2 diabetes, and trachoma (Gomes JÁ et al., 2020). Although our study did not reveal significant differences in these three microbial genera between the groups, our findings lend additional support to their potential pivotal role in sustaining ocular homeostasis.

The ocular surface microbiota in healthy individuals is typically stable and exhibits low diversity (Ozkan and Willcox, 2019). However, in metabolic diseases such as diabetes, there is a significant increase in the alpha diversity of ocular surface microbiota (Li et al., 2019). Our study observed a trend towards increased alpha diversity and a substantial rise in OTUs among patients with obesity. Furthermore, the pronounced differences in beta diversity between patients with obesity and healthy individuals indicate a distinct ocular surface microbiota composition in those with obesity. Noteworthy fluctuations at the genus and OTUs levels, marked by an increase in Cutibacterium and Delftia, as well as the discovery of 205 unique OTUs in the obese group, further affirm the presence of a specialized core microbiome in those affected by obesity. These insights reveal that the standard makeup of the ocular surface microbiota is subject to significant modifications in light of metabolic shifts, which may jeopardize the stability of the ocular surface’s ecological balance.

Notably, the abundance of Cutibacterium (Propionibacterium), previously associated with severe meibomian gland dysfunction (Watters et al., 2017) and identified as a core microbiota component in dry eye syndrome (Andersson et al., 2021), was markedly higher in patients with obesity. Considering the known link between obesity and a heightened risk of ocular conditions like meibomian gland dysfunction and dry eye syndrome (Cheung and Wong, 2007; Osae et al., 2019), our findings raise the possibility that ocular surface microbiota could play a role in the onset and progression of obesity-related ocular diseases, though further studies are necessary to establish this connection.

In their research on chronic cicatrizing conjunctivitis (CCC), Di Zazzo et al. discovered that an overactive CYP450 system might trigger inflammation in CCC by signaling lipid mediators, including arachidonic acid metabolites and lysolecithin (Di Zazzo et al., 2020). Our study’s functional predictions indicate significant enrichment of CYP450 and lipid metabolism pathways, particularly those involving arachidonic acid and glycerophospholipids, in patients with obesity. Arachidonic acid, via CYP450 metabolic pathways, can produce various signaling lipids, while glycerophospholipids can generate lysolecithin through phospholipase catalysis (Di Zazzo et al., 2020). These results imply that abnormal lipid metabolism in patients with obesity could contribute to chronic ocular inflammation mediated by CYP450. However, due to the absence of targeted metabolomics in ocular tissues and the complexity of CYP450 metabolic mechanisms, additional research is required to verify these findings.

Chronic inflammation in the eye may disrupt the ocular barrier, contributing to T cell infiltration, the loss of conjunctival goblet cells and epithelium, apoptosis, and the secretion of inflammatory cytokines. This process potentially increases vascular permeability and impairs the normal function of the ocular barrier, possibly altering the ocular surface microbiota (Mantelli and Argüeso, 2008). Our study revealed significant reductions in ECM-related pathways in patients with obesity, while pathways associated with the NOD-like receptor signaling and bacterial infections were notably enriched. The ECM, comprising the basement membrane and interstitial matrix, is vital for the adhesion, differentiation, and growth of epithelial and goblet cells, and it maintains the mechanical and structural integrity of the eye (Makuloluwa et al., 2023). Its diminished pathways indicate a compromised ocular barrier in obesity. NOD-like receptors, known for their critical role in ocular inflammation as pathogen-associated molecular pattern receptors, were also a focus of our study. An mRNA analysis of 110 patients with corneal ulcers showed heightened expression of NOD-like receptor protein 3 (NLRP3) and increased levels of inflammatory cytokines such as interleukin-1β (IL-1β), IL-8, and IL-17 (Karthikeyan et al., 2011). Similar trends in NLRP3 gene expression and IL-1β levels were observed in patients with dry eye syndrome (Zheng et al., 2015). In addition, our study also found that the abundance of the genus Delftia, which is closely related to corneal infection and postoperative endophthalmitis, was relatively higher in patients with obesity (Dantam et al., 2020; Deb et al., 2020). We hypothesize that the ocular surface microbiota in obesity may exacerbate ocular inflammation through the NOD-like receptor signaling pathway, leading to a compromised ocular barrier and a heightened risk of ocular diseases. Future research should focus on enhancing the detection of ocular inflammatory cytokines and conducting ocular biopsies and animal experiments to substantiate these hypotheses.

Additionally, previous studies have found that gender also affects the ocular surface microbiome (Wen et al., 2017). Interestingly, we did not observe gender-induced microbial differences in the obese group, whereas clear differences were evident in the healthy group. These findings lead us to hypothesize that the influence of obesity on the ocular surface microbiota may outweigh gender differences. In future studies focusing on the ocular surface microbiota, it may be necessary to regard obesity as a significant factor and incorporate it into the analysis.

While this study’s significant findings provide valuable insights, it’s worth noting the limitations to better appreciate the context and potential of the research. The study’s relatively small sample size and regional focus, along with the younger age of the participants, may limit broader applicability. Additionally, the functional forecasts provided by PICRUSt2 tend to focus on established functionalities and fail to capture the full complexity of microbial communities at the strain level. However, it sets a strong foundation for future research, suggesting the benefit of larger-scale, multi-center studies that encompass diverse age groups and ethnicities to further validate and expand upon these findings. Such studies should also incorporate metagenomics and metabolomics to further validate and extend these findings. Although the study does not establish a direct causal link between ocular surface microbiota and obesity or related eye diseases, it opens new avenues for research. Future investigations, including animal experiments and mechanistic studies, are poised to build upon this groundbreaking work, delving deeper into the mechanisms at play. In essence, this study serves as a catalyst for more comprehensive research.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: https://www.ncbi.nlm.nih.gov/, PRJNA1044261.

The studies involving humans were approved by Ethics Committee of Henan Provincial People’s Hospital. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

CL: Writing – original draft, Writing – review & editing. LW: Writing – original draft, Writing – review & editing. XW: Writing – original draft, Writing – review & editing. YJ: Data curation, Writing – review & editing. QX: Writing – review & editing. LZ: Writing – review & editing. HY: Writing – original draft, Writing – review & editing.