Clinical response to probiotic therapy Assessments will be made at baseline and at day 28 of therapy. The three symptoms of IBS will be assessed to determine response to therapy: abdominal pain/ discomfort, bloating/ distension and bowel movements, each scored on an ordinal scale (Likert scale; maximum score 7) and on a 100mm visual analogue scale (VAS; maximum score 100). A composite score with the sums of the 3 cardinal symptoms will also be calculated for each patient (Likert scale maximum score 21) (VAS; maximum score 300). Repeat rectal sensitivity testing will be performed.