Recent studies have shown that gut microbes are increasingly closely related to NAFLD[16–18]. Regarding liver diseases, the comparison of germ free (GF) and conventional (CV) mice has been used to assess the role played by the microbiota. Gut microbiota transplantation in GF mice has also been used to assess the causality between microbiota composition and susceptibility to NAFLD[9, 19]. Le Roy et al. demonstrated that gut microbiota composition determines NAFLD development in C57BL/6 strain mice for the first time. In fact, by transplanting the gut microbiota of mice with or without NAFLD into GF mice, it was found that the propensity to develop NAFLD traits could be transmitted via the gut microbiota[9, 20]. However, there are differences between the intestinal microbiota of NAFLD model mice and NAFLD patients. Therefore, it is particularly important to realize the colonization of the intestinal microbiota from NAFLD patients in germ-free mice, while striving to mimic the intestinal environment of NAFLD patients to the fullest extent possible. Chien-Chao Chiu et al[21] demonstrated that colonization of fecal bacteria from patients with non-alcoholic steatohepatitis (NASH), as well as HFD feeding, exacerbated disease progression in NAFLD. Nevertheless, the research conducted on the influence of intestinal microbiota colonization on disease progression in mice models of NAFLD patients remains insufficient and incomplete. In this study, we colonized the gut microbiota from healthy volunteers and NAFLD patients into germ-free mice and found that the liver histology and blood composition of the mice in the NAFLD group were similar to those of NAFLD disease. This indicates that there is great potential to establish a mouse model that mimics human NAFLD-related microbiota transplantation by optimizing the microbiota transplantation method.
Notably, we noticed a significant increase in serum 1-Methylxanthine in mice in the NAFLD group. Yan GAO et al [22] reported that 1-methylxanthine was significantly positively correlated with IL-6 and TNF-α when an inflammatory response occurs in the body. In patients with NAFLD, metabolic disorders including insulin resistance and obesity further promoted liver inflammation and tumorigenesis through IL-6 and TNF-α[23]. Zahra Safari et al[9] reported that the abundance of clostridium innocuum in the gut microbiota of patients with NAFLD was significantly higher than that in healthy people. Our study has validated a marked positive correlation between the significant elevation of 1-Methylxanthine in the serum of mice inoculated with gut microbiota from NAFLD patients and the substantial increase in the abundance of clostridium in their guts (Fig. 5). This finding indicates that clostridium may hold a pivotal role in the onset and progression of NAFLD. Similarly, there are still many potential links between the closely related differential microorganisms and differential metabolites in Fig. 5 that need to be further explored.
Although there are important discoveries revealed by the current work, there are certain limitations. First, the similarities and differences between the gut microbiota of NAFLD patients and the colonized mouse gut microbiota were not further revealed. This is necessary for us to further understand the relationship between the dominant bacterial genera that promote the progression of NAFLD in humans and mice. Second, the relationship between serum metabolites in mice and serum metabolites in patients with NAFLD is unclear. In future research work, we will explore these questions more deeply.
The interactions between the gut and liver, called the gut–liver axis, play an essential role in NAFLD development and evolution. Portal blood flow connects the intestine to the liver[24–26]. A significant portion of liver blood originating from the intestine subjects the liver to various metabolic products generated by the gut microbiome, encompassing phenols, acetaldehyde, and ammonia, in addition to pro-inflammatory bacterial components like peptidoglycan and lipopolysaccharides (LPS)[27]. Therefore, it is necessary to explore the potential association between serum metabolites and gut microbiota, in order to uncover further insights into the pathogenesis of NAFLD or discover an effective treatment for NAFLD. We sincerely anticipate that this study may shed new light on the search for a treatment for NAFLD.