Enterotype refers to clusters composed of gut microbial communities with similar bacteria composition, was first proposed in 2011(11), which is stratification of human gut microbiota essentially. Previous studies have found that the Bacteroides enterotype was an independent risk factor for type 2 diabetes (12). Due to the lack of a unified standard for the method of enterotype analysis, researchers should choose the method suitable for their own samples. In this study, the composition of gut microbiota can be simply described as Bacteroides enterotype and Prevotella enterotype. But not as before, in this study participants were defined with Prevotella/Bacteroides > 0.01 as Prevotella enterotype, which has a high relationship with DOR. Dietary habit is strongly linked with enterotype, other factors such as lifestyle, environmental pressure, age, and antibiotics are also related with enterotype (13). As mentioned earlier, we gave Bifidobacteria to individuals with Bacteroides enterotype. For individuals with Prevotella enterotype, we supplemented with polysaccharide to reduce Enterobacteriaceae and improved the indigestion of protein to reduce butyrate. The ovarian reserve function in both groups was significantly improved.
Gut dysbiosis has been shown to be associated with a variety of diseases, including inflammatory bowel disease, Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, obesity, diabetes and so on(14). In this study, Prevotella was the dominant taxa in DOR group, while Bacteroides was significantly abundant in the normal group. These results are consistent with Hiroyuki Sasaki et al. They found that Prevotella-enriched microbiota was present in the irregular menstrual cycles group, whereas Bacteroidales S24-7 was significantly less abundant(3).
DOR affects ~ 10% of infertile women with a decreased number and quality of oocytes, low oocyte retrieval, poor embryo quality, high cycle cancellation rate, and low clinical pregnancy rate during in vitro fertilization-embryo transfer (IVF-ET) treatment. Total mural granulosa cell apoptosis, which may be used as a marker to indicate oocyte quality, is increased in women with DOR, resulting in worse ovarian response and fewer oocytes (15). Mutation of GDF9 gene plays an important role in DOR by disturbing ovarian hormone secretion such as inhibin B, affecting folliculogenesis and granulosa cells proliferation (16). The increase of inflammatory cytokines such as TNF-α and IL-6, and the decrease of anti-inflammatory cytokines such as IL-4, IL-10 play important roles in premature ovarian insufficiency (17). In addition, Coulam et al. found that pathologic biopsy of part of the ovarian tissue of premature ovarian failure patients showed follicular inflammatory cell infiltration (18). However, none of these studies provided early prediction of DOR, which continues to confound reproductive experts. This study is first to comprehensively compare gut microbiota between DOR patients and healthy individuals. In this study, gut microbiota was thought to participate in DOR through the following mechanism. Gut dysbiosis enhances the permeability of the intestinal barrier, releases endotoxin into the blood, and promotes the production of pro-inflammatory cytokines. Disorder of digestion and absorption of nutrients produces harmful metabolites, which are involved in the development of disease. Gut microbiota is involved in the development of DOR by regulating the gut-brain axis and interfering with the Hypothalamus-Pituitary-Ovary axis by producing SCFAs. The results confirmed the hypothesis that gut dysbiosis contributes to the occurrence and development of DOR through the decreased beneficial bacteria and increased disease-related bacteria.
In DOR patients, Clostridium leptum and Lactobacillus were both positively correlated with AMH, which is a commonly used clinical indicator to reflect ovarian reserve function. He et al. discovered that administration of Lactobacillus as prebiotics could not only alleviate ovary abnormalities of letrozole-treated polycystic ovary syndrome (PCOS) rats, restore LH, FSH and testosterone levels, but also improve gut dysbiosis such as decreases the abundance of Prevotella and increases the abundance of AKK(19). The current study also yielded consistent results, Clostridium leptum and Lactobacillus are beneficial bacteria could exhibit health-promoting effects through participating in the maintenance of gut microbiota balance and producing SCFAs which can improve gut health, increase the intestinal barrier function, and reduce the translocation of bacterial endotoxins(20).
In DOR patients with the Bacteroides enterotype, AKK and Bifidobacteria were both positively related to AMH. Zhang et al. found that Bifidobacteria, as a kind of probiotics, could significantly reduce the total testosterone level in women with PCOS, which might be related to the reconstruction of gut microbiota, the enhanced digestion, and absorption of nutrients in the diet and the interaction in the gut-brain axis(21). AKK attached to the intestinal mucosa, using the mucin as a source of energy, inhibiting the colonization of pathogens through competition effect (22) and reducing endotoxin levels(23). These two beneficial bacteria can prevent the development of DOR in population with Bacteroides enterotype by improving the integrity of the intestinal barrier, reducing the level of endotoxin (24), and reducing the expression of inflammatory genes in intestinal epithelial cells (25). Among SCFAs, butyrate, as the preferred energy source of intestinal epithelial cells (26), has received particular attention for its beneficial effects. It can increase colon motility and promote water and sodium absorption. In an animal model, butyrate enhances the intestinal barrier function by regulating tight junction proteins (27). However, this study had contradictory results since butyrate was positively correlated with DOR. This discrepancy may be due to the different concentrations caused the opposite effect of butyrate. High circulating concentrations of propionate and butyrate were observed in acidaemic disease in humans (28), indicating that SCFAs might exhibit toxicity at high concentrations. Some studies have shown that butyrate can induce the development of obesity through involving in glycolipid metabolism signal pathways. Metabolic syndrome was one of the possible mechanisms of DOR(29).
In DOR patients with Prevotella enterotype, AKK and Enterobacteriaceae were negatively related with AMH. Some scholars hypothesized that AKK can transform mucin into beneficial substances to maintain the balance of host intestinal microorganisms(30). There was no evidence proving that AKK alone had a special pathogenicity in people. It might be pathogenic in the presence of other pathogenic bacteria or in susceptible people, which was consistent with the findings obtained in this study. Enterobacteriaceae as harmful bacteria, whose abnormal elevation may damage the intestinal barrier and increase intestinal permeability by stimulating the secretion of pro-inflammatory cytokines, directly interfering with the tight junctions of epithelial cells, or triggering neutrophils to migrate through the intestinal epithelium(31). Taken together, the dominant Prevotella bacteria and the abnormally elevated Enterobacteriaceae are both Gram-negative bacteria, whose lipopolysaccharide could enter circulation through the damaged intestinal mucosa, promoting the development of metabolic endotoxemia and inflammatory responses(32). In summary, in the existence of abnormally elevated Enterobacteriaceae, high levels of AKK showed adverse health effects, which may explain the opposite effect of AKK in the two enterotypes.
In this study, isobutyrate, as by-products of protein degradation, was markedly higher in patients with the Prevotella enterotype. Healthy individuals displayed a lower amount of isobutyrate compared to colorectal cancer patients(33). In our daily clinical practice, we found that patients with the Prevotella enterotype, the harmful bacteria was significantly increased after taking protein powder. However, the harmful bacteria decreased after stopping taking protein powder for a period of time. Because of the diet preference of high carbohydrate and fiber, people with the Prevotella enterotype could not digest protein easily. Hence, a high-protein diet for a long time might be one of the predispositions to DOR in people with the Prevotella enterotype through producing overmuch isobutyrate. However, after identification of enterotypes for patients, different food supplementation was offered. Enterobacteriaceae and isobutyrate decreased in patients with the Prevotella enterotype, and Bifidobacteria increased in patients with Bacteroides enterotype. The AFC of six patients was significantly higher than before and entered the hyperstimulation cycle after the intervention.
This study revealed the connection between gut microbiota and DOR and helped to establish the subsequent intervention plan. At present, most studies on gut microbiota used 16S rDNA sequencing technology, whereas this study used qPCR technology. Compared with 16S rDNA technology, qPCR technology could not accurately determine the microbiota to the species level. qPCR technology is more suitable for clinical use because of its convenience and low price. It can also be quantitative, which can be used for judging the process of disease and evaluating the clinical intervention effect. There are still some limitations in this study: (1) Sample taken from a rectal swab can also be different from a stool sample and (2) Small sample size. And although diet is the main determinant of gut microbiota, the onset of DOR is a combination of gene, environment and lifestyle. Therefore diet is also an important part and needs to be taken into account, there is no need to strip away the effects of diet. Despite the above limitations, this study revealed the potential relationship between DOR and gut microbiota, and different intervention plans were designed for different enterotypes with significant effects. Further large-scale prospective cohort studies will provide increased insights into this problem. Further work is needed to assess the potential relationship and its mechanism.