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The clinical effect of probiotics on irritable bowel syndrome (IBS) is still controversial.We aimed to evaluate the effects of a probiotic mixture on IBS symptoms and the composition of fecal microbiota in patients with diarrhea-dominant IBS (D-IBS).Fifty patients with D-IBS were randomized into placebo or probiotic mixture (Lactobacillus acidophilus, Lactobacillus plantarum, Lactobacillus rhamnosus, Bifidobacterium breve, Bifidobacterium lactis, Bifidobacterium longum, and Streptococcus thermophilus 1.0×1010 CFU) groups. Treatment was taken daily for 8 weeks. The primary outcome was adequate relief (AR) of overall IBS symptoms, which was assessed weekly for 10 weeks. A responder was defined as a patient who experienced AR for at least half of the 10-week study period. Secondary outcomes included the effects on individual symptoms, stool parameters, and IBS quality of life. The fecal flora compositions were analyzed by polymerase chain reaction denaturing gradient gel electrophoresis (DGGE).The proportion of AR was consistently higher in the probiotics group than in the placebo group throughout the 10-week period (P<0.05). The proportion of responders was significantly higher in the probiotics group than in the placebo group (48% vs. 12%, P=0.01). Stool consistency improved significantly in the probiotics group compared with the placebo group. Percent changes in individual symptom scores were similar in the 2 groups, but IBS quality of life improvement tended to be higher in the probiotics group. Comparison of denaturing gradient gel electrophoresis profiles of fecal flora showed that the concordance rate between bacterial compositions before and after treatment was significantly higher in the probiotics group than in the placebo group (69.5% vs. 56.5%, P=0.005).The probiotic mixture was effective in providing AR of overall IBS symptoms and improvement of stool consistency in D-IBS patients, although it had no significant effect on individual symptoms. The therapeutic effect of probiotics is associated with the stabilization of intestinal microbiota.