Gingivitis is an inflammatory condition of the gingival tissue, most commonly caused by bacterial infection. Among all the periodontal diseases, gingivitis is considered to be the commonest. Gingivitis, as a significant public health issue, threatens thousands of people worldwide, imposing a considerable economic and health burden on society (Peres et al., 2019). The gut microbiota constitutes the largest microbial habitat in the human body, playing a pivotal role in metabolic and immunological functions. Consequently, any alterations in the gut microbiota can potentially lead to significant systemic repercussions (Boulange et al., 2016).

Recent studies have revealed that gut inflammation involves multiple pathways, with the gut microbiome playing a significant role in health (He et al., 2021). In the aspect of oral health, researcher proposed a concept of the ‘oral-gut axis’, emphasizing the interconnectedness between oral and gut microbiota (Chen B.Y. et al., 2023; Ferrillo et al., 2023). This interaction may also play a role in the crosstalk of systemic inflammation complications mediated by gingivitis (Van Dyke et al., 2020). Specifically, pathogenic organisms linked to gingivitis may impact the composition of the gut microbiota through the continuous swallowing of saliva, thereby influencing systemic diseases (Lam et al., 2023). On the other side, alterations in gut microbiota induced by systemic diseases are often concomitant with changes in oral microbiota and localized gingivitis lesions, affecting the host immune response (Byrd and Gulati, 2021).

Recent years have seen an exploratory study uncovering a connection between gut microbiota and gingivitis. Irrespective of periodontal conditions, a significant number of oral taxa associated with periodontal destruction and inflammation have been identified in the gut microbiomes of individuals (Lourenvarsigmao et al., 2018). Furthermore, gum therapy has been shown to effectively alleviate inflammatory symptoms in patients with gingivitis and associated systemic diseases, including notable improvement in those with liver cirrhosis (Bajaj et al., 2018). Some non-surgical periodontal therapy (NSPT) methods, such as incorporating oral probiotics, have been proposed as adjuncts to subgingival instrumentation. The administration of probiotics has shown beneficial effects, albeit limited, on clinical and microbiological outcomes in the management of Gingivitis patients (Ozener et al., 2023). Despite the limited clinical evidence supporting them, these initial studies underscore the importance of gut microbiota in gingivitis.

In the medical and therapeutic context, it’s essential to determine if the link between gut microbiota and gingivitis is merely correlational or driven by pathogenic mechanisms. Despite extensive research on epidemiology and pathophysiology, the causal association remains unclear due to reverse causality and other confounding effects. For example, experimental research has highlighted the role of Bacteroides in exacerbating gingivitis (Caselli et al., 2020). However, this stands in contrast to observational studies that propose the anti-inflammatory effects of Bacteroides metabolites, presenting a complex narrative (Luo et al., 2023). To navigate these complexities, MR offers a unique approach. MR employs the correlation between disease and genotype to simulate the influence of exposure factors on the disease, introducing genetic variations related to these factors as instrumental variables (Birney, 2022). Due to its temporal validity and capability to minimize confounding factors, Mendelian randomization is regarded as a supplementary approach to randomized controlled trials (Larsson et al., 2023).

In this study, we performed a two-sample MR analysis using publicly available genome-wide association study (GWAS) databases. Our goal was to investigate the potential causal relationship between gut microbiota and gingivitis, offering genetic evidence for the significance of gut microbiota in gingival health.

In this investigation, a two-sample MR analysis was conducted to assess the potential causal association between gut microbiota and gingivitis. Our findings suggest an intricate interaction between gut microbiota and gingivitis. Specifically, three microbial were identified with an elevated risk of gingivitis, while four exhibited an association with a diminished risk. Our research findings bridge the knowledge gap regarding the potential role of gut microbiota in gingivitis and delineate specific taxonomic groups that may either facilitate or hinder the onset of gingivitis.

The correlation between gut microbiota and gingivitis has long been a subject of research interest (Byrd and Gulati, 2021). Traditional research methodologies face inherent challenges in fully elucidating the intricate relationship between gut microbiota and gingivitis. This complexity stems from the dynamic nature of both oral and gut microbiota, influenced by multifaceted factors such as diet, lifestyle, genetics, environment, medication, and disease. Moreover, the precise causal direction and underlying mechanisms governing the bidirectional influence of oral and gut microbiota, along with their impact on the inflammatory processes within the oral and intestinal mucosa, remain incompletely understood. Consequently, observational studies are susceptible to confounding variables and reverse causation, while experimental approaches may not capture the comprehensive temporal and spatial variations in microbiota and host response (Smith and Ebrahim, 2003). Investigating host genetic variation stands forth as a compelling and pivotal research domain (Chen Q. et al., 2023). To overcome these limitations, some researchers have proposed and applied novel methods to investigate the causal relationship between gut microbiota and gingivitis, such as Mendelian randomization, metabolomics, and machine learning. These methods can leverage the genetic and metabolic data of the microbiota and the host, and use statistical and computational techniques to infer the causal effects and interactions of the microbiota on the disease outcomes.

It is noteworthy that we discovered novel taxa with positive associations that have not been reported in prior literature. The study revealed a higher abundance of the genus Defluvitataeaceae on the tonsil surface in the hypertrophic group compared to the healthy group, indicating a potential association with tonsillar inflammation (Xu et al., 2021). Defluviataleaceae, like Defluvitataeaceae, also falls within the order Defluviatales; however, it is distinct from Defluvitataeaceae based on phylogenetic and phenotypic characteristics. It has been mentioned that some Defluviataleaceae bacteria can degrade aromatic compounds, such as phenol and benzoate, which are known to induce oxidative stress and inflammation in the brain (Talebi et al., 2021). Through our MR analysis, it can be concluded that reducing the number of family Defluviitaleaceae and genus Defluviitaleaceae UCG011 is beneficial for controlling gingivitis.

Utilizing whole-genome sequencing and bioinformatics analysis, the study identified that Bacteroidia, encompassing the order Bacteroidales, demonstrated notable diversity and abundance in the oral cavity (Caselli et al., 2020). Based on preceding investigations, it was noted that Bacteroidales represented one of the taxa exhibiting a marked adverse effect on gingivitis (Luo et al., 2023). However, conflicting outcomes emerge from an observational study, suggesting the anti-inflammatory properties of metabolites originating from the Bacteroidales group (Luo et al., 2023). Indeed, our MR study can overcome these challenges by providing validation from a genetic perspective, presenting clear evidence that the class Bacteroidia group contribute to the risk of gingivitis.

In addition to the previously mentioned three taxa promoting gingivitis, we also identified four gut microbiota taxa negatively associated with gingivitis, which are reported for the first time. Despite previous studies associating Porphyromonadaceae with inducing inflammation and tissue damage (Eisenstein, 2021), our investigation revealed that Porphyromonadaceae exhibits anti-inflammatory effects. The inconsistency among studies could be attributed to high individual variations in gut microbiota composition and the multifaceted nature of inflammatory diseases (Eckburg et al., 2005). Next, an article highlighted Escherichia Shigella and Ruminococcaceae UCG002 as generals that exhibited a significant increase in the intestinal microbiota of these patients with decompensated alcoholic cirrhosis. Additionally, it emphasized that short-chain fatty acids (SCFAs), produced by certain gut bacteria, may modulate the inflammatory response and liver function (Baltazar-Diaz et al., 2022). In conjunction with existing findings, our MR study suggests that preventive measures and control of gingivitis may be attainable by increasing the abundance of the genus Escherichia Shigella and genus Ruminococcaceae UCG002 through various interventions. A study underscored the significance of actinomycetes in the synthesis of bioactive compounds, including antibiotics, anticancer agents, anti-inflammatory agents, and enzymes (Selim et al., 2021), which may be the mechanisms of class Actinobacteria to reduce risks of gingivitis.

Considering the direct connection between the oral and gastrointestinal tracts and the extent to which disturbances in the oral microbiome affect the gut microbiome and subsequently gut stem cells, such as Lgr5, contributing to the systemic disease (e.g. liver cancer) development, remains unresolved (Barker and Clevers, 2007; Han et al., 2021; He et al., 2023), investigating the interaction between these microbiotas is of significant relevance (Uchiyama et al., 2019). Elucidating the specific contribution of distinct gut microbial taxa to gingivitis holds potential for enhancing prevention and control strategies. Despite efforts to understand the association between gingivitis and gut microbiota, no evidence supporting a causal effect has been proposed. Furthermore, the dysbiosis phenotype observed in the gut microbiota of gingivitis patients results from multifactorial influences, and the strain-specific changes in various microbiota taxa are inconsistent. Variability in the composition of the gut microbiota can be attributed to disparities in the staging of gingivitis, gender distribution, and ethnic composition across different study populations. These factors present challenges in establishing a specific causal relationship between gut microbiota taxa and the risk of gingivitis. Further, Oral disease treatment has been proven effective in alleviating inflammatory symptoms in patients with periodontitis and associated systemic diseases, notably including improvements in cirrhosis patients (Bajaj et al., 2018).This effectiveness prompts the investigation of whether such treatments could also impact other systemic disease (e.g. liver cancer) progression by modifying underlying biological pathways, potentially guiding the development of targeted periodontal interventions for the disease prevention. In short, the pivotal role of gut microbiota in gingivitis and overall health underscores the potential for investigating targets along the “oral-gut axis” to manage inflammatory disorders, which may involve immunological approaches to regulate intestinal microbiota (Zhou et al., 2023).

As stated earlier, MR is an optimal study design for assessing causal relationships between potential risk factors and diseases. Recently, multiple MR studies have probed nuanced risk factors associated with gingivitis. This study from Frontiers in Genetics used MR to examine the causal relationship between circulating cytokines and gingivitis. It found that interleukin 9 (IL9) had a positive causal relationship with gingivitis, and interleukin 17 (IL17) had a negative causal relationship with gingivitis (Huang et al., 2023). Through the examination of factors influencing gingivitis risk, MR studies inform the development of public health policies and clinical interventions to effectively reduce its incidence and societal burden. Certain factors previously implicated in gingivitis through epidemiological observational studies were found to lack a causal association with gingivitis according to MR studies, such as depression and psoriasis (Nolde et al., 2022; Yin et al., 2022). In contrast to earlier MR studies, our investigation is more comprehensive, delineating the causal impact of 418 gut microbiome taxa on gingivitis. Previous MR studies concentrated on fewer than 10 exposures of interest. We conducted a replication analysis to verify the robustness of the MR estimates using two independent datasets (IEU OpenGWAS and FinnGen), greatly enhancing the credibility of our results.

This study represents the forefront of MR investigations, utilizing extensive genetic data on gut microbiome and gingivitis to assess the potential causal relationship between gut microbiota and the risk of gingivitis. A notable strength of our study is the robust application of the MR method, effectively mitigating concerns related to reverse causal associations and confounding factors. Additionally, our MR study encompasses a broad population range at a relatively low cost, offering practical and compelling insights compared to conventional observational studies. Nonetheless, this study has some limitations that need to be acknowledged. Firstly, the majority of the GWAS participants were of European descent, which may limit the applicability of our results to other populations. Secondly, considering the biological plausibility and intricate pathobiology of gingivitis, along with the multifaceted nature of statistical processes, applying a stringent multiple-testing correction may be excessively conservative. Such an approach could potentially overlook strains that are partially causally correlated with gingivitis. Consequently, we decided not to implement multiple corrections (Zhang et al., 2022). Thirdly, our study targeted the elucidation of risk factors for gingivitis with the goal of facilitating comprehensive clinical intervention and reducing the incidence. Consequently, our focus was on examining the unidirectional influence of 418 gut microbiota taxa on gingivitis. Fourth, the precise mechanisms by which the aforementioned gut microbiota taxa influence the risk of gingivitis have not been comprehensively investigated in this study.

In conclusion, this study is the first to demonstrate a causal relationship between gut microbiota and gingivitis using MR analyses. It reveals the impact of specific gut microbiota taxa on the susceptibility to gingivitis, offering new perspectives for designing clinical interventions to prevent and treat gingivitis.

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

ZH: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Project administration, Resources, Software, Validation, Visualization, Writing – original draft. CR: Writing – original draft. LS: Conceptualization, Funding acquisition, Methodology, Supervision, Validation, Writing – review & editing.