Background  A multispecies probiotic has shown beneficial effects in irritable bowel syndrome. In addition, certain other probiotics have demonstrated advantageous effects, but the mechanisms behind this are poorly understood.

Aim  To investigate the mode of action of a multispecies probiotic consisting of Lactobacillus rhamnosus GG, Lactobacillus rhamnosus Lc705, Propionibacterium freudenreichii ssp. shermanii JS and Bifidobacterium breve Bb99 by monitoring its effects on intestinal microbiota and markers of microbial activity.

Methods  A total of 55 irritable bowel syndrome patients participated in this placebo-controlled double-blind trial. Subjects received either multispecies probiotic or placebo supplementation daily during a 6-month period. The composition of intestinal microbiota was analysed with real-time polymerase chain reaction, short-chain fatty acids with gas chromatography and enzymes with spectrophotometer.

Results  Each supplemented probiotic strain was detected in faecal samples. Intestinal microbiota remained stable during the trial, except for Bifidobacterium spp., which increased in the placebo group and decreased in the probiotic group (P = 0.028). No changes in short-chain fatty acids occurred. A decrease in ß-glucuronidase activity was detected in 67% of the subjects in the probiotic group vs. 38% in the placebo group (P = 0.06).

Conclusions  Factors other than the microbial groups and metabolites studied herein seem responsible for the alleviation of irritable bowel syndrome symptoms by the multispecies probiotic.