RETRACTED ARTICLE: Role of the gut microbiome in chronic diseases: a narrativereview

The gut microbiome, i.e., the community of bacteria and other microorganisms living in the human gut, has been implicated both directly and indirectly (mediating the effects of diet) on human health [ 1 , 2 ]. The associations between gut microbiome composition and disease status have been widely reported, while recent studies have demonstrated a role for the gut microbiome in influencing remote organs, mucosal, and immune function [ 3 , 4 ]. Considerable effort is currently focused on understanding the natural history of microbiome development in humans in the context of health outcomes, in parallel with improving our knowledge of microbiome–host molecular interactions. These efforts ultimately aim to develop effective approaches to rehabilitate perturbed human microbial ecosystems as a means to restore health and prevent disease. This review details the role of the gut microbiome in chronic diseases (Fig. 1 ) and ways it can be modulated for the management or prevention of chronic conditions. Fig. 1: Schematic representation of the association of the composition of the gut microbiome and gut-derived metabolites with chronic diseases. The solid lines represent negative associations and the dashed lines represent positive associations of the disease phenotype with gut microbes and metabolites. Full size image Differences in gut microbiome composition and function have been associated with a variety of chronic diseases ranging from gastrointestinal inflammatory and metabolic conditions to neurological, cardiovascular, and respiratory illnesses. The aim of this narrative review is to describe the associations between gut microbiome composition and various types of chronic diseases and to discuss the links to habitual diet and dietary components (Table 1 ). Table 1 Summary of key findings outlining the role of the gut microbiome in chronic disease. Full size table Gut microbiome and autoimmune disease The pathogenesis of autoimmune diseases (AIDs) is not only attributed to genetic susceptibilities but also environmental factors, among which, a disturbed gut microbiota has attracted increasing attention. Compositional and functional changes of gut microbiota have been reported in various autoimmune diseases, and increasing evidence suggests that disturbed gut microbiota contributes to their immunopathogenesis. Rheumatoid arthritis (RA) Rheumatoid arthritis (RA) is a systemic autoimmune inflammatory condition that manifests in joint damage. It was recently demonstrated that different environmental factors are involved in the development of both intestinal/oral dysbiosis and arthritis onset and outcome, among which the most relevant are diet, smoking, and infections [ 5 , 6 ]. The observation that germ-free mice are protected from development of experimental arthritis [ 7 ] suggests a possible role for the microbiome in the pathogenesis of this disease. The composition of the gut microbiota in RA patients free of therapy is severely altered compared to healthy controls. Chen et al. reported that compared with healthy controls, patients with RA show decreased gut microbial diversity, which correlates with autoantibody levels and disease duration [ 8 ]. In addition, at the compositional level, patients with RA show an increased abundance of Prevotella species, including Prevotella copri [ 8 , 9 ], and recent preclinical phase studies on RA patients in European countries were shown to harbor a high abundance of this species in the intestine, suggesting that dysbiosis precedes the development of arthritis [ 10 ]. In contrast, Faecalibacterium , which is generally recognized as a beneficial microbe, is decreased in RA patients. Furthermore, the relative abundance of Collinsella was found to be increased in RA patients [ 11 ]. Interestingly, inoculation of Collinsella into collagen-induced arthritis (CIA)-susceptible mice induces severe arthritis. In vitro experiments showed that Collinsella Aerofaciens increases gut permeability and induces IL-17A expression, a key cytokine involved in the pathogenesis of RA, suggesting that Collinsella is a candidate arthritogenic bacterium in the human intestine [ 11 ]. In summary , Prevotella copri [ 12 ] and Collinsella are the dominant gut microbiota in patients with early RA and may be involved in its pathogenesis. Recent links have been made between dietary intake of short-chain fatty acids (SCFAs) and autoimmune arthritis in mice, wherein SCFAs play an important role in the suppression of inflammation in RA [ 13 , 14 ]. Mice deficient for SCFA receptors showed exacerbated inflammation in modes of RA [ 14 ]. Butyrate, one of the most abundant SCFAs, acts as an endogenous histone deacetylase (HDAC) inhibitor and has been shown to decrease inflammation in animal models of RA and other inflammatory diseases [ 15 ]. A recent study has revealed a role for intestinal barrier function, and specifically for zonulin, a peptide that controls epithelial tight junction permeability, in regulating the onset of joint disease in mice with collagen-induced arthritis (CIA) and potentially also in patients with RA [ 16 ]. Increased levels of zonulin have been associated with leaky intestinal barrier, dysbiosis, and inflammation. Restoration of the intestinal barrier in the period before clinical arthritis, either by dietary supplementation with the SCFA butyrate or pharmacological agents such as a zonulin antagonist may help delay disease onset and reduce the severity of RA. Type 1 diabetes (T1D) In humans, intestinal microbiota alterations, including loss of bacterial diversity, preceded the onset of metabolic symptoms associated with T1D [ 17 ]. Previous studies have linked several facets of gut health with the onset of T1D in humans and animal models [ 18 , 19 , 20 ]. In animal models, antibiotic-induced dysbiosis, altered microbial lipid metabolism, and suppressed enteric Th17 and T-reg cell populations have led to increased incidence of T1DM-like disease in mice [ 21 ].