Advances in the Understanding of the Intestinal Micro-Environment and Inflammatory Bowel Disease
Inflammatory bowel disease (IBD) is a chronic, relapsing inflammatory disorder of the gastrointestinal tract, primarily encompassing ulcerative colitis (UC) and Crohn’s disease (CD). The pathogenesis of IBD is complex, involving genetic susceptibility, environmental factors, and dysbiosis of the gut microbiota. Over the past decade, significant advances have been made in understanding the role of the intestinal micro-environment in IBD, particularly through the application of multi-omics technologies and microbiome modulation strategies. This review aims to summarize recent findings on the dysbiosis of the intestinal micro-environment, the interplay between bacterial and fungal microbiota, and the clinical application of probiotics and fecal microbiota transplantation (FMT) in IBD.
Dysbiosis of the Micro-Environment in IBD
The gut microbiota plays a crucial role in maintaining intestinal homeostasis, and its dysbiosis is strongly associated with the pathogenesis of IBD. Dysbiosis in IBD is characterized by a reduction in microbial diversity and an imbalance in the composition of the microbiota. Several factors, including diet, antibiotics, and bowel preparation for colonoscopy, can disrupt the intestinal micro-environment. Host genetics, mucosal transcription levels, and metabolic products are the primary contributors to species-based microbiota taxonomy, while disease status and diet account for a smaller portion of microbiota structure variation.
In patients with IBD, the decline in bacterial diversity and load has been well-documented. However, functional changes in the intestinal microbiota are less understood. Recent studies have shown that disturbances in metabolites, such as short-chain fatty acids (SCFAs), are associated with IBD pathogenesis. SCFAs, including acetate, propionate, and butyrate, are essential for maintaining energy homeostasis and regulating mucosal immune responses. Faecalibacterium prausnitzii, a butyrate-producing bacterium, is significantly reduced in patients with CD and UC, and its abundance inversely correlates with disease activity. Despite these findings, the relationship between specific bacterial species and metabolite concentrations remains inconsistent, highlighting the need for more comprehensive models to evaluate intestinal micro-environment dysbiosis.
Mycobiome and Its Interplay with the Microbiome
While the bacterial microbiome has been extensively studied in IBD, the role of the fungal microbiome, or mycobiome, is less understood. Fungi constitute less than 1% of the total microbial population in the gut, making their identification challenging. However, recent studies using 18S rRNA and internal transcribed spacer (ITS) sequencing have revealed distinct fungal microbiota configurations in patients with IBD. Certain fungi, such as Blastocystis and Saccharomyces, are associated with microbial richness and diversity, and their prevalence is reduced in IBD patients.
The interplay between bacterial and fungal microbiota is crucial for maintaining intestinal homeostasis. Fungal microbiota can counteract bacterial dysbiosis and reduce inflammation in acute colitis. However, in chronic colitis, fungal microbiota may exacerbate inflammation and translocate to other organs, such as the spleen and mesenteric lymph nodes. The interaction between Candida tropicalis, Serratia marcescens, and Escherichia coli has been shown to promote the formation of antimicrobial-resistant biofilms in patients with CD. Additionally, Candida albicans produces farnesol, a quorum-sensing molecule that modulates mucosal immune responses and influences the virulence of other pathogens.
The Emerging Field: Dynamic Alteration of Multi-Omics
Traditional single-omics approaches have provided valuable insights into the gut microbiota but fail to capture the complexity of IBD pathogenesis. Multi-omics approaches, which integrate data from genomics, transcriptomics, proteomics, and metabolomics, offer a more comprehensive understanding of the intestinal micro-environment. These approaches have been used to identify biomarkers for disease severity and treatment response in IBD.
For example, multivariate models based on transcriptomics and metabolomics data have identified candidate biomarkers for UC severity. Similarly, multi-omics analysis has revealed impaired mitochondrial hydrogen sulfide detoxification functions and upregulated hydrogen sulfide-producing bacteria in patients with CD. Atopobium parvulum, a sulfur-metabolizing bacterium, has been identified as a central hub in the dysbiotic network of CD patients. The colitogenic effects of A. parvulum have been demonstrated in IL-10-deficient mice, and its effects can be mitigated by hydrogen sulfide scavengers.
Multi-omics studies have also explored the interplay between host genetics and the gut microbiota. For instance, genetic variants associated with bacterial handling, such as CARD9 and SLC39A8, have been linked to specific microbiota shifts in IBD patients. These findings highlight the potential of multi-omics approaches to uncover novel therapeutic targets and mechanisms in IBD.
Application of Microbiome Modulation in IBD
Given the critical role of the gut microbiota in IBD pathogenesis, microbiome modulation has emerged as a promising therapeutic strategy. Probiotics, prebiotics, and FMT are among the most studied interventions for microbiome modulation in IBD.
Probiotics
Probiotics are live microorganisms that confer health benefits when administered in adequate amounts. They exert their effects by maintaining the intestinal barrier, modulating mucosal immunity, and regulating the micro-environment. Several probiotic formulations, such as Bifico and VSL#3, have shown efficacy in inducing and maintaining remission in IBD patients.
Bifico, a probiotic cocktail containing Bifidobacterium longum, Lactobacillus acidophilus, and Enterococcus faecalis, has been shown to enhance the efficacy of mesalazine in inducing remission in patients with mild to moderate CD and UC. Long-term follow-up studies have demonstrated a lower recurrence rate in patients receiving Bifico compared to those on mesalazine monotherapy. Additionally, Bifico has shown chemopreventive effects in colitis-associated cancer models by altering the mucosal microbiota and downregulating pro-inflammatory cytokines.
VSL#3, another probiotic mixture containing multiple Lactobacillus and Bifidobacterium species, has been widely studied in UC patients. Clinical trials have shown that VSL#3 can induce remission in patients with mild to moderate UC and improve disease activity scores. However, the efficacy of VSL#3 in CD remains controversial, with some studies reporting higher relapse rates in patients receiving VSL#3.
Fecal Microbiota Transplantation (FMT)
FMT involves the transfer of fecal microbiota from a healthy donor to a patient with dysbiosis. FMT has shown promise in inducing remission in IBD patients, particularly those with UC. Clinical remission rates of 52% and 33% have been reported in CD and UC patients, respectively, following FMT. Responders to FMT tend to exhibit a more significant shift in microbiota composition, with increased abundance of beneficial bacteria such as Fusobacterium and Enterobacteriaceae.
The efficacy of FMT may vary depending on the route of administration. Studies comparing upper gastrointestinal (via gastroscopy) and lower gastrointestinal (via colonoscopy) routes have found no significant differences in clinical remission rates. However, the optimal FMT regimen, including the number of treatments and donor selection criteria, remains to be determined.
Summary
The intestinal micro-environment plays a pivotal role in the pathogenesis of IBD, with dysbiosis of the gut microbiota being a key factor. Advances in multi-omics technologies have provided deeper insights into the complex interactions between the host, microbiota, and environmental factors. Microbiome modulation strategies, including probiotics and FMT, have shown promise in the treatment of IBD. However, further research is needed to standardize these interventions and identify patient subgroups that may benefit the most. The integration of multi-omics data with clinical outcomes holds the potential to revolutionize the management of IBD, paving the way for personalized medicine approaches.
doi.org/10.1097/CM9.0000000000000718
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