Previous studies have established a connection between MDD and IS at the population level, but the specific mechanisms that explain the co-occurrence of MDD and IS have not been identified. Due to the close connection between the gut microbiota and these two illnesses, we hypothesized that the gut microbiota may also play a critical role in the development of IS in individuals with MDD. To elucidate the mechanisms associated with the gut microbiota, we analyzed the gut microbiota and metabolome in MDD patients with IS and MDD patients without IS. The present study showed through gut microbiota and metabolomic analyses that there are significant changes in the makeup of the gut microbiota community and metabolites in MDD patients with IS and MDD patients without IS. Our research provides a fresh point of view on the association between MDD and IS.
In the examination of the taxonomic composition disparity between the two groups, we observed that at the phylum level, the proportion of Proteobacteria was greater in IS patients than in non-IS patients. A Proteobacteria increase is considered an indicator of microbial dysbiosis[23]. Prior studies have demonstrated that individuals with MDD exhibit disrupted gut microbiota features, although there is much heterogeneity in the methods and reporting of these studies[16, 17, 24, 25]. Moreover, our study showed that MDD patients with IS exhibit more severe disruptions in their gut microbiota. The elevated abundance of Proteobacteria can be ascribed to the enrichment of Enterobacteriaceae, and these findings align with a previous investigation conducted on individuals with IS during both the acute and recovery phases[20]. Enterobacteriaceae are bacteria characterized by their LPS structures. LPS is a crucial constituent of the outer membrane's external layer in gram-negative bacteria. It can trigger an immune response and induce inflammation in the host[26–28]. The presence of a high level of Enterobacteriaceae in the intestines, in conjunction with an impaired gut barrier, promptly triggers widespread inflammation throughout the body by producing a substantial quantity of LPS. This process may contribute to the development of IS, and higher levels of LPS are associated with worse outcomes in patients with IS[20, 29–31]. In addition, the abundance of Veillonella parvula, which is a gram-positive bacterium that possesses an outer membrane with LPS similar to that of gram-negative bacteria, was similarly greater in IS patients than in non-IS patients[32]. The distributions of SCFA-producing bacteria in the gut microbiota of patients with IS and those without IS were ultimately distinct. Patients with IS exhibited elevated levels of propionic acid-producing bacteria, including Veillonella parvula, Alloprevotella, and two subspecies of Prevotella (Prevotella_sp_Marseille_P2931 and Prevotella_stercorea). Conversely, there was a lower abundance of butyric acid-producing bacteria, such as Acidaminococcaceae, Roseburia, and Fusicatenibacter. These findings suggest that various SCFAs have distinct functions in the progression of IS in individuals with MDD. Prior studies have demonstrated that individuals with acute IS with diabetes have decreased levels of bacteria that produce butyric acid[33]; furthermore, even those without a previous history of stroke but with a high risk of experiencing one also showed a reduction in the level of butyric acid-producing bacteria[34], implying that bacteria that produce butyrate may play a protective role in the development and prognosis of IS. The aforementioned results indicate that the gut microbiota of patients with MDD during the recovery phase after IS is similar to that of patients with acute-phase IS. The primary characteristics of this gut microbiota include an increase in the presence of bacteria with LPS structures and a decrease in the quantity of bacteria that produce butyrate.
Bacteroides massiliensis is a bacterium that is found at relatively low abundances in people with IS. Prior research examining the gut microbiota of patients with cancer, including prostate cancer[35], colorectal cancer[36], and melanoma[37], revealed that the abundance of Bacteroides massiliensis was greater in patients with tumors. However, it has also been shown that individuals with diabetes and those with diabetes-related cardiovascular issues have significantly lower levels of Bacteroides massiliensis than healthy individuals[38]. This finding demonstrates the complexity of the relationship between Bacteroides massiliensis and disease. Additional investigations are needed to determine whether Bacteroides massiliensis contributes to the development of IS in patients with MDD.
Metabolomes represent the combined effects of internal physiological processes and external factors. The human gut microbiota engages in significant interactions with the host through the cometabolism of substrates and the exchange of metabolites[39]. Consequently, we conducted an analysis of the patients' plasma metabolomics. We observed the considerable impact of the presence or absence of IS on the metabolites of these strains. We discovered two distinct compounds, daidzein and glucobrassicin, that are strongly linked to the presence of two specific types of bacteria, Alloprevotella and Bacteroides massiliensis. Daidzein is a type of flavonoid compound that is found in soybeans and several soy-based products and exists in the form of glucosides[40]. The gut microbiota, which includes Bacteroidetes, Firmicutes, Entererococcus, Lactobacillus, and Bifidobacterium, contains genes that encode various glycosidase enzymes, such as β-glucosidase. These enzymes can breakdown and transform soy sapogenins, which are glycosides, into free sapogenins and glucose[41]. Once separated, the free sapogenins undergo a range of reactions, including dihydroxylation, reduction, pyrone ring cleavage, or demethylation. These events result in the formation of a new molecule that exhibits potent estrogenic activity, such as equol, or a product that is inactive, such as O-desmethylangolensin[40, 42]. Equol has demonstrated significant promise in the prevention of cardiovascular disease (CVD). Equol may decrease the likelihood of developing coronary heart disease by enhancing its anti-atherogenic properties, improving arterial stiffness, and reducing LDL-C levels in overweight people[43]. Glucobrassicin, an important secondary metabolite found in cruciferous vegetables, can be broken down to form indole-3-methanol (I3C). Both I3C and its primary in vivo derivative, 3,3’-diindolylmethane (DIM), have demonstrated efficacy as cancer chemopreventive agents in preclinical models and have shown potential in clinical trials. I3C also exhibited significant neuroprotective effects in a rat model of clonidine-induced depression[44]. Similarly, I3C has been shown to demonstrate significant protective or therapeutic effects against diabetes[45, 46], hypertension[47, 48], and a rat model of cerebral ischemia/reperfusion[49]. These effects may be attributed to its antioxidant activity and capacity to inhibit the inflammatory response. While previous research has relied on animal tests, the current study revealed that individuals with MDD and IS had reduced levels of glucobrassicin. These findings suggested that glucobrassicin may have a protective effect on patients with MDD complicated with IS. Moreover, there was an inverse relationship between daidzein and glucobrassicin with Alloprevotella, while a positive relationship was found with Bacteroides massiliensis. These findings suggested that daidzein and glucobrassicin may have an impact on patients with MDD by interacting with these two bacteria. However, further investigation is required to determine the exact mechanism of action involved. Ultimately, the random forest analysis identified Alloprevotella and Bacteroides massiliensis as reliable discriminators between IS patients and non-IS patients. Nevertheless, we found that the addition of related metabolites can significantly enhance the accuracy of the prediction. Hence, the gut microbiota and metabolites may also prove beneficial in the prompt detection of patients with MDD who have IS.
We also acknowledge various limitations of this study. Initially, the hospital's lockdown as a result of the COVID-19 pandemic made sample collection more challenging. The IS patients we collected were in the recovery phase rather than the acute phase. However, previous studies have shown a significant increase in the abundance of Enterobacteriaceae in patients with acute and recovering IS[20]. Consequently, our research can offer a reference regarding the composition of the gut microbiota in patients with MDD during the acute stage of IS. Furthermore, 16S rRNA sequencing lacks the ability to provide genetically modified annotations that are precise enough to identify the specific species. Therefore, shotgun metagenomic sequencing should be performed for further microbiome analysis.