Background:Irritable bowel syndrome (IBS) is reported associated with the alteration of gut microbial composition termed as dysbiosis.However,the pathogenic mechanism of IBS remains unclear,while the studies of Chinese individuals are scarce.This study aimed to understand the concept of dysbiosis among patients with Chinese diarrhea-predominant IBS (IBS-D),as a degree of variance between the gut microbiomes of IBS-D population and that of a healthy population.Methods:The patients with IBS-D were recruited (assessed according to the Rome Ⅲ criteria,by IBS symptom severity score) from the Outpatient Department of Gastroenterology of Peking University Third Hospital,and volunteers as healthy controls (HCs) were enrolled,during 2013.The 16S rRNA sequences were extracted from fecal samples.Ribosomal database project resources,basic local alignment search tool,and SparCC software were used to obtain the phylotype composition of samples and the intal interactions of the microbial community.Herein,the non-parametric test,Wilcoxon rank-sum test was carried out to find the statistical significance between HC and IBS-D groups.All the P values were adjusted to q values to decrease the error rate.Results:The study characterized the gut microbiomes of Chinese patients with IBS-D,and demonstrated that the dysbiosis could be characterized as directed alteration of the microbiome composition leading to greater disparity between relative abundance of two phyla,Bacteroidetes (Z =4.77,q =1.59 × 10-5) and Firmicutes (Z =-3.87,q =5.83 × 10-4).Moreover,it indicated that the IBS symptom features were associated with the dysbiosis of whole gut microbiome,instead of one or several certain genera even they were dominating.Two genera,Bacteroides and Lachnospiracea incertae sedis,were identified as the core genera,meanwhile,the non-core genera contribute to a larger pan-microbiome of the gut microbiome.Furthermore,the dysbiosis in patients with IBS-D was associated with a reduction of network complexity of the interacted microbial community (HC vs.IBS-D:639 vs.154).The disordered metabolic functions of patients with IBS-D were identified as the potential influence of gut microbiome on the host (significant difference with q ＜ 0.01 between HC and IBS-D).Conclusions:This study supported the view of the potential influence of gut microbiome on the symptom of Chinese patients with IBS-D,and further characterized dysbiosis in Chinese patients with IBS-D,thus provided more pathological evidences for IBS-D with the further understanding of dysbiosis.