In the MR analysis, leveraging GWAS data on gut microbiome composition and female reproductive health conditions, our research elucidates causal relationships of genetic susceptibility to specific gut microbiota with inflammatory pelvic disorders and infertility in women. Employing stringent criteria for the selection of IVs, we systematically assessed the associations between 119 bacterial genera and a range of female reproductive diseases. We identified 45 SNPs in 4 genera associated with inflammatory disorders of female pelvic organs and 83 SNPs in 7 genera related to female infertility. Notably, subanalysis revealed distinct microbiota implicated in different organs involved in inflammation or infertility. Each bacterial genus, with its unique metabolic activity influencing immune modulation [27, 28], was found to either increase or mitigate disease risk. Multiple analytical methods were employed to ensure the robustness of the associations, including diverse MR approaches, Cochran Q-tests, MR‒Egger regression intercept analyses, and MR-PRESSO analyses. Importantly, our study navigated the pitfalls of weak IV bias, identifying potential horizontal pleiotropy and outliers, thereby enhancing the study's reliability and depth.
Previous research has indicated a complex relationship between gut microbiota and both inflammation and infertility [8, 29–33]. These investigations have primarily focused on correlational analyses and lack causal insights. To our knowledge, the present study is the first to systematically explore the causal relationship between gut microbiota and female reproductive tract inflammation and infertility by employing MR methodology. Specifically, we found that certain bacterial genera, such as Ruminococcus 2, are not only generally associated with an increased risk of developing inflammatory disorders in female pelvic organs but also show a heightened correlation with specific conditions, such as salpingitis and oophoritis, in suborgan analyses. Furthermore, Ruminococcus 2 is linked to an increased risk of developing vaginal or vulvar inflammation. In other diseases associated with inflammation, such as IgA nephropathy and rheumatoid arthritis [34, 35], Ruminococcus 2 is notably enriched in the gut microbiota. Additionally, in the context of other obstetric and gynecological conditions, Ruminococcus 2 has also been observed to be significantly associated with gestational diabetes mellitus [36], or preeclampsia [37]. This underlines the marked relevance of Ruminococcus 2 in relation to reproductive health.
Furthermore, we observed that the Butyricicoccus genus had a protective role against inflammatory disorders of female pelvic organs. This anaerobic bacterial genus primarily resides in the gut microbiota and produces short-chain fatty acids, particularly butyrate [38, 39], which has various biological activities, such as anti-inflammatory and immunomodulatory effects [40, 41]. Correspondingly, numerous studies have demonstrated that Butyricicoccus exerts a protective role in various inflammatory diseases [39, 42, 43]. Intriguingly, some bacterial genera play both protective and detrimental roles in various pelvic inflammatory disorders. For instance, Prevotella 9 appears to reduce the risk of developing uterine inflammatory disorders while posing a risk of developing vaginal or vulvar inflammation. These dual roles are also reflected in other MR analyses; Prevotella 9 is shown to mitigate the risk of developing psoriasis [44], while potentially increasing the risk of developing gestational diabetes mellitus [36, 45]. Additionally, Prevotella 9 has been positively correlated with programmed cell death protein-1 [46]. Further studies have also indicated that the abundance of Prevotella in the vagina serves as a hub of infection leading to cervical lesions in women of reproductive age [47]. Additionally, Prevotella 9 had a protective effect against anovulation-related infertility, underlining the intricate interactions between specific gut bacteria and varying organ locations, as well as hinting at a potential link between reproductive inflammation and infertility. In infertility analyses, most cases of female infertility were related to ovulatory disorders, uterine or cervical issues, or fallopian tube alterations. Increasing evidence confirms that dysbiosis in the gut microbiota has an indispensable impact on inflammatory conditions affecting both male and female fertility [48, 49]. Observational studies have found that women with infertility display different microbial abundances and proportions, as well as increased levels of inflammation [49].
Lactobacillus, one of the nine core microbial genera, has been extensively documented to play a protective role in female reproductive health by metabolizing glycogen from vaginal epithelial cells into lactic acid. This biochemical activity not only establishes a mildly acidic environment conducive to beneficial vaginal bacteria but also inhibits harmful bacteria through the production of antimicrobial substances [50–53]. Conversely, Faecalibacterium prausnitzii, the most abundant bacterium in the healthy adult human gut, constitutes more than 5% of the total bacterial population and has been implicated as a promising therapeutic agent in inflammatory bowel disease and nephritic diseases [54–56]. However, our research intriguingly identifies Faecalibacterium as a potential risk factor for infertility, thereby highlighting unexplored mechanisms deserving of further investigation.
While the specific roles of certain bacteria in the gut microbiota remain not fully elucidated, their influence on reproductive health can be contemplated from several vantage points. Initially, the gut microbiota holds the potential to modulate systemic immune responses, thereby possibly affecting the status of the endometrial lining and subsequent fertility outcomes[57, 58]. Additionally, specific signaling molecules generated by the gut microbiota may exert regulatory effects on the reproductive tract and other reproductive organs, influencing pivotal physiological processes such as ovulation and implantation[59, 60]. Gut bacteria also play a role in the metabolic pathways of hormones, including estrogen, thereby having a potential bearing on female reproductive health[60]. Moreover, imbalances in the gut microbiota may precipitate systemic inflammation, which could, in turn, implicate the reproductive system, manifesting in conditions such as oophoritis or endometritis[61, 62]. Vaginal inflammation or infections may also correlate with imbalances in the gut microbiota, subsequently affecting fertilization and implantation[63]. Furthermore, the gut microbiota is posited to influence psychological states, and prolonged psychological stress is conjectured to adversely affect fertility[64, 65]. Last, the gut microbiota is implicated in nutrient absorption, and deficiencies in trace elements and vitamins could have repercussions on fertility[66–68]. Concurrently, the gut microbiota's impact on body weight and metabolism should not be overlooked, as weight anomalies—be they obesity or underweight—are established to be detrimental to fertility[49, 69–71].
The current study holds several unique advantages. By conducting MR analysis to determine the causal relationship between gut microbiota and reproductive inflammation as well as infertility, the study effectively mitigates the influence of confounding variables and potential reverse causality. Genetic variants associated with gut microbiota were gathered from the most extensive GWAS meta-analyses, thereby fortifying the robustness of the IVs utilized in the MR analysis. In addition to analyzing general pelvic organ inflammation and infertility, we also explored different organ tissues specifically. Multiple statistical methodologies were employed to ensure consistency of the results. Techniques such as MR-PRESSO and MR‒Egger regression intercept tests were used to detect and eliminate horizontal pleiotropy. Furthermore, reverse MR analysis was utilized to ascertain the presence of any reverse causality. Through a two-sample MR design and the utilization of nonoverlapping exposure and outcome summary-level data, biases were minimized, setting a stringent framework for exploring the complex relationship between gut microbiota and reproductive health [72].
However, there are limitations worth noting that could potentially influence the interpretation of our findings. Most patients in the GWAS summary data utilized in our study were of European descent, while only a minimal amount of gut microbiota data originated from other ethnic groups, potentially leading to biased estimations and affecting generalizability. The GWAS meta-analysis for gut microbiota was not restricted to female participants [17]. Although genetic variants located on sex chromosomes were excluded, and gender was adjusted for in the analyses [17], potential sex-specific biases could not be entirely ruled out. Bacterial taxa were analyzed only at the genus level; utilizing more advanced shotgun metagenomic sequencing in GWASs could yield more specific and accurate results [14]. Furthermore, we did not delve deeper into exploring the relationship between reproductive inflammation and infertility. Additionally, the relatively small sample size and the inclusion of SNPs that did not meet the traditional GWAS significance threshold (P < 5×10− 8) could introduce bias [73]. To fully understand the causal relationship between gut microbiota and reproductive health, future MR studies should address these limitations, possibly by incorporating a more diverse population and refining the taxonomic levels considered in the analysis. Future research should adopt an integrative approach using multiple omics platforms to enhance our understanding of disease pathogenesis in the complex backdrop of gene‒environment interactions.