Characterization of gut microbiomes in nonalcoholic steatohepatitis (NASH) patients: a connection between endogenous alcohol and NASH

Microbiota and gut-liver axis: their influences on obesity and obesity-related liver disease

A specific bacterial gut microbiota profile with increased extraction of energy has recently been associated with obesity, which has been shown to be a transmissible phenotype by microbiota transplantation. At the same time, there is now increasing evidence that gut microbiota plays a role in the development of hepatic steatosis and its progression to nonalcoholic steatohepatitis. This review summarizes well known and unexpected interacting factors leading to obesity and its related hepatic diseases, including intestinal mucosal permeability and its regulation, gut microbiota and translocation of its biological products, and gut-associated lymphoid tissue. These intestinal factors dictate also the balance between tolerance and immune response, which are critical for most of the complications in near and far organs or systems. We review novel mechanisms involving the development of gut permeability and adipose tissue plasticity, for example, the cross-talk between the gut microbiota, lipopolysaccharide, high-fat diet, and the endocannabinoid system tone, which have not been fully explored. Interactions between gut microbiota and other factors (eg, inflammasome deficiency) also are reviewed as emerging but far from being completely elucidated mechanisms influencing the onset of obesity and nonalcoholic fatty liver disease.

Linking intestinal homeostasis and liver disease

PURPOSE OF REVIEW

Interactions of the gut microbiome with the host are important in health and disease. Microbial translocation releases bacterial products that play a key role in progression of chronic liver disease by promoting hepatic injury and inflammation. Although this has long been recognized, we are just beginning to understand the circumstances under which the gut becomes leaky and to discover bacterial metabolites that promote liver disease. In this review, we will summarize recent findings from the last 2 years.

RECENT FINDINGS

Chronic liver disease is associated with an altered microbiome with both qualitative (dysbiosis) and quantitative (overgrowth) differences. This can be viewed as a loss of the symbiotic relationship between the microflora and the host. An imbalanced intestinal homeostasis results in a breach of the gut barrier and subsequent microbial translocation. However, the contribution of the intestinal microflora is beyond simple microbial translocation as a pathogenic factor. Bacterial metabolites resulting from an imbalanced homeostasis and dysbiosis play also a crucial role in liver disease.

SUMMARY

A combination between an initiating liver insult and a disturbance of the gut-host symbiosis synergize in progression of liver disease.

Molecular characterization of the fecal microbiota in patients with nonalcoholic steatohepatitis--a longitudinal study

Background

The human gut microbiota has profound influence on host metabolism and immunity. This study characterized the fecal microbiota in patients with nonalcoholic steatohepatitis (NASH). The relationship between microbiota changes and changes in hepatic steatosis was also studied.

Methods

Fecal microbiota of histology-proven NASH patients and healthy controls was analyzed by 16S ribosomal RNA pyrosequencing. NASH patients were from a previously reported randomized trial on probiotic treatment. Proton-magnetic resonance spectroscopy was performed to monitor changes in intrahepatic triglyceride content (IHTG).

Results

A total of 420,344 16S sequences with acceptable quality were obtained from 16 NASH patients and 22 controls. NASH patients had lower fecal abundance of Faecalibacterium and Anaerosporobacter but higher abundance of Parabacteroides and Allisonella. Partial least-square discriminant analysis yielded a model of 10 genera that discriminated NASH patients from controls. At month 6, 6 of 7 patients in the probiotic group and 4 of 9 patients in the usual care group had improvement in IHTG (P = 0.15). Improvement in IHTG was associated with a reduction in the abundance of Firmicutes (R² = 0.4820, P = 0.0028) and increase in Bacteroidetes (R² = 0.4366, P = 0.0053). This was accompanied by corresponding changes at the class, order and genus levels. In contrast, bacterial biodiversity did not differ between NASH patients and controls, and did not change with probiotic treatment.

Conclusions

NASH patients have fecal dysbiosis, and changes in microbiota correlate with improvement in hepatic steatosis. Further studies are required to investigate the mechanism underlying the interaction between gut microbes and the liver.

Nonalcoholic fatty liver disease: for better or worse, blame the gut microbiota?

Nonalcoholic fatty liver disease (NAFLD) is a major clinical consequence for people with obesity and metabolic syndrome and is also associated with enteral and parenteral nutrition. Early studies suggested that altered gut microbiota might contribute to obesity by affecting energy harvest from the diet and energy storage in the host. Recent evidence in humans as well as in animal models has linked gut microbiota to the development of NAFLD through the gut-liver axis. With bacterial overgrowth and increased intestinal permeability observed in patients with NAFLD and in animal models, gut-derived bacterial products such as endotoxin (lipopolysaccharide) and bacterial DNA are being delivered to the liver through the portal vein and then activate Toll-like receptors (TLRs), mainly TLR4 and TLR9, and their downstream cytokines and chemokines, leading to the development and progression of NAFLD. Given the limited data in humans, the role of gut microbiota in the pathogenesis of NAFLD is still open to discussion. Prebiotics and probiotics have been attempted to modify the microbiota as preventive or therapeutic strategies on this pathological condition. Their beneficial effects on NALFD have been demonstrated in animal models and limited human studies. However, prospective, appropriately powered, randomized, controlled clinical trials are needed to determine whether prebiotics and probiotics and other integrated strategies to modify intestinal microbiota are efficacious therapeutic modalities to treat NALFD.