A.muciniphila was first isolated and cultured in vitro by researchers in 2004. As an intestinal symbiotic bacteria, it accounts for a considerable proportion of the intestinal microflora of mammals. Studies have found that the abundance of A.muciniphila in the intestines of obese and type 2 diabetes patients was significantly reduced, while the administration of A.muciniphila to mice on a high-fat diet alleviated obesity, reduced weight gain, and improved blood glucose levels and insulin resistance. Studies have also shown that the diabetes drug metformin improved glucose metabolism in HFD mice, to some extent, by increasing the abundance of A.muciniphila in the gut. A series of studies suggested that A.muciniphila was a highly beneficial intestinal symbiont. However, researchers found that, in some type 2 diabetes, colon cancer and Parkinson's patients, the abundance of A.muciniphila increased, suggesting that A.muciniphila might play a part in these diseases and might be a harmful bacterium[19, 26-30]. There was no detailed explanation for what led to completely different, or even opposing, conclusions, as experts stated that whether a microbe is helpful or harmful is highly dependent on a multitude of host and microbial factors, including the dose, strain, duration of colonization, and specific application of the microbe; and the age, genetic background, environmental factors, disease status, and host process in question, and here our hypothesis was that this discrepancy might be related to different A.muciniphila genotypes.
We had previously isolated 39 strains of A.muciniphila from the human intestine, and, using high-throughput sequencing, divided them into three genotypes, of which the type strain ATCC-835 belongs to A.muciniphila I . Consistent with previous studies, our study found that A.muciniphila abundance was significantly reduced in obese people (BMI＞30), with a significant decrease in A.muciniphila I and A.muciniphila II. In 2014, Shin et al.  demonstrated that metformin could improve glucose homeostasis in HFD mice by increasing the abundance of A.muciniphila in the gut. However, we found that, in type 2 diabetic patients treated with metformin, the abundance of A.muciniphila I increased obviously while A.muciniphila II did not change significantly, which led us to speculate that the roles of A.muciniphila I and II might be different. In the present study, we found that, in HFD mice, A.muciniphila I and II could improve the impaired glucose tolerance and promote the release of insulin to an extent. The effects of A.muciniphila II on regulating glucose metabolism were not obvious, however, suggesting that A.muciniphila gavage could ameliorate glucose metabolic disorders in mice, but that the effects of A.muciniphila I were more pronounced than A.muciniphila II .
Recent studies have shown that the lipid-lowering drugs ezetimibe, fenofibrate, and rusvarastine inhibit the progression of non-alcoholic steatohepatitis(NASH), indicating the role of hyperlipidemia in NASH. Currently, there is a shortage of drugs to treat NASH correspondingly, therefore necessitating precautions against NASH. Our experiments found that A.muciniphila gavage significantly decreased the serum TG and TC levels in HFD mice, with A.muciniphila I being more effective in TG regulation, while A.muciniphila II was more effective in modulating TC levels. Additionally, we found that A.muciniphila gavage significantly inhibited the accumulation of fat in the liver while improving hyperlipidemia in HFD mice. However, only A.muciniphila I showed statistically significant effects. Compared with TC, TG should be a more appropriate independent risk factor for fatty liver. Studies have shown that serum TG was a significant risk factor for fatty liver, while TC was predominantly correlated with coronary atherosclerotic cardiopathy. Given that A.muciniphila II had a more remarkable effect on TC, A.muciniphila II may act more strongly in regulating cholesterol metabolism. Meanwhile, A.muciniphila I reduced serum TG levels notably, which may have promoted the improvement of fatty liver.
A detailed analysis in 2015 showed that the abundance of A.muciniphila was positively correlated with BAT marker gene levels in HFD mice. In 2017, Gao  proved that A.muciniphila administration could directly promote the browning of WAT. However, this study was performed by treating A.muciniphila culture medium, and therefore could not exclude the interference of A.muciniphila metabolites in the culture medium, and this study did not focus on changes in BAT itself. Studies have shown that BAT happened to be whitened in cases of obesity, presenting as multilocular adipocytes gradually replaced by unilocular adipocytes, increased lipid droplets in BAT, decreased numbers of mitochondria and UCP1 expression, infiltrating inflammatory factors, and decreased levels of BAT marker genes . Other studies found that BAT knockout in mice could lead to obesity, that reduced BAT-dependent non-shivering thermogenesis could damage insulin sensitivity, and the expression of UCP1 in WAT effectively reduced weight[35, 36]. Several findings suggested that BAT whitening and BAT dysfunction might induce or even exacerbate obesity and related metabolic syndromes. In this study, it was demonstrated that the scapular BAT was significantly whitened in HFD mice, and the thermogenic activity was decreased. Strikingly, after A.muciniphila treatment, although we did not observe significant browning of iWAT and eWAT, the number of unilocular adipocytes decreased significantly in BAT, UCP1 expression increased and other BAT marker gene levels were upregulated. We found that A.muciniphila can inhibit BAT whitening and maintain BAT activity in HFD mice, although the effects of A.muciniphila II were slightly inferior to A.muciniphila I. In addition to obesity, aging can also lead to BAT whitening [37, 38]. It is worth mentioning that we observed mild BAT whitening in the NCD mice, which might be due to aging and A.muciniphila I had no distinct effect. Nevertheless, A.muciniphila II could play a considerable role. These observations led us to suspect that the mechanisms of these two genotypes may be widely divergent.
In obesity, the intestinal barrier function is damaged, intestinal permeability increased, and the endotoxins secreted by gut microbiota are released into the blood, causing endotoxemia, which in turn leads to chronic systemic inflammation and further local inflammation[39, 40]. The results of this study showed that both A.muciniphila I and II effectively increased colon mucus cells and tight junction protein expression. Correspondingly, after A.muciniphila I treatment, HFD mice showed reduced serum LPS and reduced transcriptional levels of inflammatory genes in BAT, while A.muciniphila II showed no difference, suggesting that A.muciniphila I can relieve both systemic and local inflammation. Given that BAT local inflammation negatively regulates thermogenesis, this may explain why A.muciniphila I can maintain BAT activity and inhibit BAT whitening. To conclude, A.muciniphila I may improve endotoxemia induced by HFD by repairing the intestinal barrier, thereby inhibiting the development of inflammation in BAT and maintaining the activity of BAT. BAT can make use of the free fatty acid and glucose in blood as the substrate for thermogenesis, so the maintainance of BAT activity is advantageous to removing excessive free fatty acid and glucose. This, in turn, reduces blood glucose and lipid levels, improving the body's glucose and lipid metabolism (Fig. 8). This explains why A.muciniphila I gavage alleviated the impaired glucose tolerance, hyperlipidemia and liver fat accumulation in HFD mice.
It is worth noting that although A.muciniphila II could also repair the intestinal barrier, the effect of A.muciniphila II on endotoxemia was not apparent, leading to a failure of reversing BAT inflammation. Endotoxemia is not only associated with intestinal barrier function, but also with gut microbiome structure. For example, Cani proved that plasma LPS concentrations correlated negatively with Bifidobacterium spp[42, 43]. Studies have also demonstrated that high plasma LPS levels could result from an increased production of endotoxins by a change in gut microbiota[43, 44]. Despite the evidence that both of A.muciniphila I and A.muciniphila II can repair the intestinal barrier, they do not both improve endotoxemia, possibly because they possess different functions in the intestinal microecology. Despite A.muciniphila II not effectively improving BAT inflammation in HFD mice, it did function manifestly in alleviating BAT whitening and maintaining BAT activity, and acted in NCD mice (Fig. 9). It follows that the action pathways of these two genotypes may not be consistent, but further studies are needed to prove this hypothesis. Our results suggest A.muciniphila I and II exert different impacts on blood glucose and lipid metabolism, which may be related to their specific target pathways. Disparate mechanisms may determine disparate roles, which may be the cause of inconsistent evaluation of different A.muciniphila genotypes.