Background
Irritable bowel syndrome (IBS) is a chronic functional gastrointestinal disorder that affects 11% of the world’s population [
1]. IBS affects more women than men, and adults younger than 50 years of age compared with older ones [
2]. The main symptoms of IBS include abdominal pain, changes in defecation habits and/or fecal condition, abdominal distension, and discomfort [
3]. IBS imposes a large burden on patients, impairing health-related quality of life and work productively [
4]. Traditional therapeutic approaches for IBS, including dietary changes and antibiotic therapy, may not obtain satisfactory outcomes since most of them are treating symptoms. Recently, the prevalence of IBS has been rising all over the world, mainly due to anxiety and stress [
5].
The pathophysiological mechanisms underlying IBS are multifactorial and have been poorly understood. A heritable component of IBS is long recognized in family and twin studies [
6]. Evidence is now accumulating that genetic risk in IBS spans from complex polygenic conditions with combinations of common variants to cases with rare single gene abnormalities [
7,
8]. Recent studies have shown that gut microbiota may be related to the pathogenesis of IBS [
9‐
11]. Treatment with antibiotics or fecal microbiota transplantation relieves global IBS symptoms without causing constipation, suggesting a direct relationship between gut microbiota and IBS [
12,
13]. A recent systematic review has pointed out that alterations of gut microbiota exist in patients with IBS, which might exert a pivotal role in the development of IBS [
14].
Although gut microbiota has been related with IBS, the causal nature is elusive. Mendelian randomization (MR) analysis is a statistical approach that aims to infer potentially causal relationships from observational association results [
15]. MR uses genetic variants associated with exposure as a surrogate for exposure to assess the relationship between the surrogate and the outcome [
16]. In recent years, MR analysis has been applied to assess the potential causal relationships between gut microbiota and disease-risking genes [
17‐
19]. So far, there is an urgent need to investigate the potential causal relationship between gut microbiota and the risk of IBS.
In the present study, in order to explore the potential causal relationship between gut microbiota and IBS, and to identify specific pathogenic bacteria taxa, we conducted a two-sample MR study based on genome-wide association study (GWAS) summary data.
Discussion
This two-sample MR study identified a total of 11 bacterial taxa, including phylum Actinobacteria, class Melaibacteria, order Gastraerophilales and Rhodospirillales, family Rikenellaceae, and genus Eubacterium hallii group, Eisenbergiella, Flavonifractor, Coprococcus 1, Prevotella 9 and Ruminiclostridium 6, might be associated with the risk of IBS. However, sensitivity analyses using different MR methods and restricted IV sets demonstrated three bacterial taxa, Actinobacteria, Flavonifractor, and Eisenbergiella, were associated with the risk of IBS.
Phylum Actinobacteria, one of the major phyla of gut microbiota, is pivotal in the maintenance of gut homeostasis [
32]. Disorder of Actinobacteria was associated with several diseases, including inflammatory bowel disease [
33], ankylosing spondylitis [
34], and type 2 diabetes [
35]. A decrease of Actinobacteria was found in patients with IBS compared to healthy controls [
36]. The reason might be that Actinobacteria as the initial factor of IBS, the host could produce specific antibodies to reduce the abundance of Actinobacteria after IBS occurring. In addition, the abundance of Actinobacteria showed significant alterations after treatment of IBS [
37,
38]. The potential causal relationship between Actinobacteria and IBS observed in this study once again suggested the importance of Actinobacteria in the development of IBS.
Genus
Flavonifractor, a flavonoid degrader, has also been identified as a risk factor of IBS. The flavonoid compound could alleviate intestinal inflammation of IBS via macrophage-intrinsic AhR [
39]. Genus
Flavonifractor and its species
Flavonifractor plautii were enriched in the stool communities in children with IBS [
40]. In addition,
Flavonifractor plautii was correlated with recurrent abdominal pain and could elicit enhanced IgG responses in postinfectious IBS patients [
41]. Enrichment of the genus
Flavonifractor was described in adults with comorbid IBS diarrhea-predominant and depression [
42]. A previous study also suggested that dietary modifications could decrease the abundance of Flavonifractor to reduce abdominal pain or accelerated transit time in IBS [
43]. Taken together, these studies suggested that a high level of Genus
Flavonifractor may be positively associated with the risk of IBS, which is consistent with our findings.
Genus
Eisenbergiella was the only identified bacterial taxa being negatively associated with the risk of IBS in this study. However, there was no study reporting the alteration of genus
Eisenbergiella in IBS patients to date. In animal studies, only one literature reported that genus
Eisenbergiella showed an increasing trend in the IBS group compared to the control group [
44]. Even so, genus
Eisenbergiella was probably related to eubiosis because it could produce butyrate, acetate, lactate, and succinate as major metabolic products, with a trophic effect on the mucosa [
45]. Besides, genus
Eisenbergiella might be closely related to the reduction in intestinal inflammation in ulcerative colitis mice [
46]. Although this study firstly showed a potential causal relationship between genus
Eisenbergiella and the risk of IBS, further research is needed to explore the underlying biological mechanism between them.
Many previous studies showed that patients with IBS were usually accompanied by gut microbiota dysbiosis, but they were observational studies [
9,
47]. This study strengthened the causal effects of gut microbiota on IBS by using a genetic epidemiological approach. In addition, the F-statistic of IVs we used all satisfied the threshold of > 10 which suggested that our analyses were less likely to suffer from weak instrument bias. We further performed a reverse MR analysis that excluded reverse causality. Causal association research will be the future direction of studying the role of gut microbiota in the development of diseases. Nowadays, there were many kinds of research focusing on the role of certain gut bacteria in the disease development using animal models [
48,
49]. Our MR analysis results may provide a guide for selecting individual gut bacteria to study the role of gut microbiota in the pathogenesis of IBS.
Nevertheless, our study had several limitations. First, bacterial taxa were only analyzed at the genus level but not at a more specialized level such as species or strain levels. Second, while the majority of the participants enrolled in this GWAS are of European descent, the inclusion of participants with other ethnicities may influence the results. Consequently, the generalization of our findings to other racial groups may be subject to limitations. Third, we selected the IVs for gut microbiota at p < 1.0 × 10−5 which were larger than traditional genome-wide significance level (p < 5 × 10–8) to obtain sufficient IVs. In addition, the effect of the bacterial traits we reported was relatively weak and there was no other independent GWAS of IBS with sufficient sample size to validate our findings. Finally, since information of IBS subtypes were not available, further studies are warranted when this information become available.
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