Lower Level of Bacteroides in the Gut Microbiota Is Associated with Inflammatory Bowel Disease: A Meta-Analysis

Background and Aims. Multiple studies have reported associations between inflammatory bowel disease (IBD) and the flora disequilibrium of Bacteroides. We performed a meta-analysis of the available data to provide a more precise estimate of the association between Bacteroides level in the gut and IBD. Methods. We searched PubMed/MEDLINE, EMBASE, Cochrane Library, Wiley Library, BIOSIS previews, Web of Science, CNKI, and ScienceDirect databases for published literature on IBD and gut microbiota from 1990 to 2016. Quality of all eligible studies was assessed using the Newcastle-Ottawa Quality Assessment Scale (NOS). We compared the level of Bacteroides in IBD patients with that in a control group without IBD, different types of IBD patients, and IBD patients with active phase and in remission. Results. We identified 63 articles, 9 of which contained sufficient data for evaluation. The mean level of Bacteroides was significantly lower in Crohn's disease (CD) and ulcerative colitis (UC) patients in active phase than in normal controls. The level of Bacteroides in remission CD and UC patients was much lower than patients in the control group. Bacteroides level was even lower in patients with CD and UC in active phase than in remission. Conclusions. This analysis suggests that lower levels of Bacteroides are associated with IBD, especially in active phase.


Introduction
Inflammatory bowel disease (IBD) which includes Crohn's disease (CD), ulcerative colitis (UC), and indeterminate colitis (IC) [1] is a chronic relapsing inflammatory disorder of the gastrointestinal tract [2] of unclear etiology. One hypothesis is that the inflammation results from altered microbiota in a genetically susceptible host [3].
There are 160 major bacteria among the 1,000 to 1,150 species of bacteria which colonize the human intestinal tract. Two dominant microflora in the human distal gut, Bacteroides and Firmicutes phyla, account for 90% of the bacterial flora [4]. Microflora which play an important physiological role in the adult colon are Bacteroides fragilis, Bifidobacterium, Bacteroides, and a variety of anaerobic Gram-positive cocci. Intestinal flora in healthy individuals can demonstrate significant variety. As individuals age, their intestinal microbial flora tend to become more similar [5]. There is a healthy balance of microflora in the gastrointestinal tract in normal individuals. This balance is disrupted in disease.
Many studies have related Bacteroides to the development of IBD. Bacteroides is a Gram-negative, nonspore forming, obligate anaerobic bacteria normally found in the human intestines, mouth, upper respiratory tract, and genital tract [6]. Bacteroides expresses polysaccharide A, which can induce regulatory T cell growth and cytokine expression that are protective against colitis [7].
We performed a meta-analysis of case-control studies to assess role of Bacteroides in IBD.
search strategies were used and keywords were used in preference to MeSH terms to increase the sensitivity of our search. The following key words were used: inflammatory bowel disease, IBD, Crohn's disease, CD, ulcerative colitis, UC, indeterminate colitis, IC, flora disequilibrium, and Bacteroides.
An expanded search was done using Google Scholar and by contacting authors of selected articles. Conference abstracts and the bibliography of selected articles were also selected to assure that no relevant studies were missed. If two or more studies shared study populations or more than one article reported the same clinical trial, the publication with more information was selected. Professors Bo Jiang and Yang Bai from the Institute of Digestive Diseases, Nanfang Hospital were contacted as the local experts in the field of gut microbiota.

Study Selection.
Studies included for analysis (a) were case-control studies; (b) had the Bacteroides level in the intestines determined; (c) were published articles or meeting abstracts; (d) compared the level of Bacteroides in patients with IBD and without IBD; (e) were published reports with enough data to analyze differences between the IBD and control groups; (f) were published in English or Chinese.
Two reviewers (YT.Z and FC.Z) independently assessed the quality of all eligible studies using the Newcastle-Ottawa Quality Assessment Scale (NOS) (http://www.ohri.ca/programs/clinical epidemiology/oxford.asp) for case-control studies. The NOS uses a "stars" rating system to judge quality including selection of the study population, comparability, and exposure assessment. Scores were ranged from 0 (the lowest) to 9 (the highest). Studies with a score ≥ 7 were considered to be of high quality. The quality of each study was awarded stars independently by the same two reviewers (Table 3).
Data was extracted from each qualified study, including the study design, first author's last name, and publication year. Discrepancies between the two authors were dealt with by a consensus meeting with all authors and discussion with our local experts.

Statistic Analysis.
Three primary analyses were performed. Different types of IBD patients' Bacteroides levels were compared with a control group. The level of Bacteroides in patients with CD, UC, and IC were also compared. Patients with active CD or UC and patients with CD or UC in remission were compared. Heterogeneity between studies was assessed using both the 2 test with a value < 0.05 and the inconsistency index ( 2 ) with a cut-off of ≥50%.
Statistical analyses were performed using SPSS version 13.0 (SPSS Corporation, Chicago, USA) and Revman version 5.0 (The Cochrane Collaboration, Oxford, UK).

Results
63 studies were initially identified. Twenty-one studies were excluded with animal experiments, twenty studies were excluded with irrelevant experiments, and eight reviews were excluded. After that, fourteen studies potentially relevant were further screened. One study was excluded because Bacteroides was acquired from the gums [10]. Four studies were excluded with data that was inappropriate for analysis [11][12][13][14].
The Bacteroides level was evaluated for 12 groups, CD versus control group, UC versus control group, IC versus control group, CD versus UC, CD versus IC, UC versus IC, active CD versus remission CD, active UC versus remission UC, active CD versus control group, and active UC versus control group. The active phase of CD was defined using a Crohn's disease activity index (CDAI) >150 and active UC using a Clinical Activity Index (CAI) >5. All patients were diagnosed by endoscopy. Continuous data and dichotomous data were analyzed separately. Bacteroides from gums (n = 1) [19] Articles with Bacteroides and IBD (n = 9) [9,10,11,12,13,14,15,16,8]
Our meta-analysis compared the level of Bacteroides in IBD patients with that in normal controls. Patients with active and inactive disease were also evaluated. A lower level of Bacteroides was demonstrated with Real-Time Quantitative PCR in CD and UC patients than in healthy controls, especially CD and UC patients with active disease. UC patients in remission also had a lower level of   Bacteroides than controls. Patients with active UC had lower Bacteroides levels than patients in remission. CD and UC patients had a higher level of Bacteroides in the gut than control patients in FISH and conventional culture studies. These methods also demonstrated a higher level of Bacteroides in the gut of CD patients than in IC patients. The level of Bacteroides in active IC patients was not reported. CD and UC patients had lower levels of Bacteroides than normal controls in Real-Time Quantitative PCR studies, but higher levels of Bacteroides than normal controls in FISH and conventional culture studies. Real-Time Quantitative PCR [22] fecal samples were mostly obtained from Asian patients, while FISH and conventional culture fecal samples were mostly from European patients. This finding suggests Bacteroides levels may be different in different ethnic groups.
Heterogeneity was found using a fixed effects model in the pooled UC versus control group, IC versus control group, and CD versus IC group. All these reports used FISH to determine Bacteroides levels. Similar findings were observed using a random effects model. Heterogeneity was attributed to two European reports, that of Sokol et al. [19] (using fecal samples) and that of Swidsinski et al. [21] (using tissue samples).
A meta-analysis is inherently limited by the studies included. In this study, many of the reports were descriptive in nature and had small sample sizes. Only two of the 6 studies reported continuous data that could be used to calculate Bacteroides levels in patients with active disease and disease in remission. Therefore, significant heterogeneity was found between the studies when data was pooled. We thought the source of the heterogeneity maybe resulted from the difference of methods to determine Bacteroides and the age. While the included publications used different methods and had small sample sizes and different patient populations, they all reached the conclusion that Bacteroides level was associated with IBD, especially in patients with active disease.
This meta-analysis of observation studies supports the finding of low Bacteroides levels in patients with IBD. Despite the differences in methods, our meta-analysis demonstrated that low levels of Bacteroides were present in IBD patients with active disease. This finding may be useful in the treatment and the etiology research of IBD patients. Prospective trials are needed to confirm the results of this meta-analysis.