Discussion
Here, we conducted a minor experiment to investigate the alteration of the gut microbiota and microbial metabolites induced by MET in HFD-induced model mice. MET treatment significantly influenced gut microbiota composition, microbial metabolites, and phenotypic indices in mice with glucolipid metabolism disorder. Further correlation analysis indicated a negative correlation between species enriched in the MET group and 2-deoxytetronic acid, as well as the phenotypic indices, and the positive correlation between species enriched in the MC group and 2-deoxytetronic acid, and the phenotypic indices. However, the correlation between the differential metabolites, which decreased after MET intervention, and the phenotypic indices was positive.
Gut microbiota and microbial metabolites have attracted increasing attention over the past decade. Researchers have conducted several experiments on the role of the gut microbiome in the pathology of diabetes and the mechanism of action of MET. A similar study reported that the therapeutic effect of MET on T2DM might occur with short-chain fatty acid production and an increase in the abundance of Escherichia species, and the restriction of the use of MET could induce the depletion of butyrate-producing species.26 The increase in the abundance of Escherichia coli after MET intervention was also observed in current study. Moreover, an experiment using tractable genetic models with a high-throughput platform found that the effects of MET on lipid metabolism could be implemented by the accumulation of microbial agmatine.27 Here, we report a novel response of MET in the intestinal environment.
Of particular interest are the bacterial taxa, such as B. vulgatus, B. fragilis, P. distasonis, and B. acidifaciens, which are known to be important in some chronic diseases.
B. vulgatus was the predominant species in the MET group. Administration of polysaccharide from Plantago asiatica L. promoted the reduction of blood glucose, insulin, TC, and TG, and increased HDL-C and B. vulgatus in high-fat diet and streptozotocin-induced type 2 diabetic rats.28 Additionally, B. vulgatus reportedly decreases in patients with coronary artery disease, and gavage with live B. vulgatus reduces gut microbial lipopolysaccharide production and suppresses proinflammatory immune responses in atherosclerosis-prone mice.29 However, several studies have indicated that B. vulgatus increases in obese people30 and is linked to low-grade inflammation and insulin resistance.31 32 These controversial results suggest that B. vulgatus can induce immunomodulatory responses that are mediated by lipopolysaccharide.33 Our study found that MET treatment increased the abundance of B. vulgatus, which may affect immune regulation. B. fragilis reportedly has anti-inflammation effects. B. fragilis produces polysaccharide A and sphingolipids, both of which are involved in the anti-inflammatory function of related T cells.34 35 In addition, B. fragilis inhibits the production of intestinal-derived corticosterone through its metabolite arachidonic acid.36 However, a contrary report indicated that MET ameliorates metabolic dysfunction by inhibiting B. fragilis, thus increasing the level of bile acid glycoursodeoxycholic acid in patients with newly diagnosed T2DM.37
P. distasonis was enriched in the MC group, and an increased abundance was observed in obese patients with T2DM in a Chinese cohort study.38 Moreover, free access to a sugar-sweetened beverage increases the abundance of P. distasonis in rats.39 However, P. distasonis may ameliorate hyperglycemia and hyperlipidemia in HFD-fed mice via its metabolites, succinate, and secondary bile acids.40 This contentious issue requires further study. B. acidifaciens markedly increased in a streptozotocin–high-fat, diet-induced non-alcoholic steatohepatitis–hepatocellular carcinoma C57BL/6 J mouse model and presented a positive correlation with bacterial lipopolysaccharide levels and pathophysiological features.41 The decreased abundance of B. acidifaciens in the MET group suggests that the therapeutic effect may be due to the amelioration of intestinal permeability and reduction of lipopolysaccharide. The results of KEGG analysis also indicated that MET increased the level of immune system, which may be associated with the gut microbial lipopolysaccharide production and regulation of the immune responses.
As for the metabolites, former studies proposed that MET regulated the production of short-chain fatty acid, butyrate, propionate,26 42 bile acid glycoursodeoxycholic acid,37 and agmatine.27 Additionally, a microbial metabolite (imidazole propionate) inhibited the effect of MET.43 In this study, we found that 2-deoxytetronic acid, the dominant metabolite correlated with gut microbiota, was depleted after treatment with MET. 2-Deoxytetronic acid is an endogenous short-chain polyhydroxymonocarboxylic acid that has been reported to be positively correlated with post-MET glucose levels in the OGTT.44 Besides, methyl jasmonate 4 may be a plant hormone based on the information from ChEBI45 (ChEBI ID:15929), which we thought might originate from the feed. Though methyl jasmonate can exert anti-inflammatory and antioxidant action,46 it was unclear why the content of methyl jasmonate 4 decreased after MET intervention. Further research applying the targeted metabolomics might better clarify the phenomenon. As for the oxamic acid, it might also come from the feed. However, high-fat and high-sugar diet can interfere with the gut microbial oxalate metabolism.47 Thus, the level of oxamic acid might decrease after MET restores the gut microbiota.
A limitation of this study is the lack of stool sample collection at the beginning of the experiment and at the end of model establishment; however, the control group may ease the influence of the previous one. More research using gavage of single species or fecal microbiota transplantation is needed to determine the regulatory network.
This study was designed to determine the effects of MET on the gut microbial system. The most obvious finding of this study is that MET may restore intestinal homeostasis and ameliorate host metabolism by inhibiting B. bacterium M6, P. distasonis, and B. acidifaciens; promoting B. vulgatus and B. fragilis; and restricting the production of 2-deoxytetronic acid. Overall, these findings suggest a role for the gut microbial system in promoting the therapeutic effects of MET on glucose and lipid regulation. The insights gained from this study may assist in fully understanding the mechanism of MET in treating metabolic diseases, especially the involvement of the gut microbiota.