Aliment Pharmacol Ther 2011; 34: 374–383

Background  Imbalances in gut luminal bacteria may contribute to the pathogenesis of irritable bowel syndrome (IBS).

Aim  To explore select bacteriological and anti‐inflammatory effects of mesalazine (mesalamine; 5‐aminosalicylic acid or 5ASA) and their relation to potential therapeutic effects in IBS.

Methods  Prospective pilot study of 12 women with diarrhoea‐predominant IBS. Patients received oral mesalazine (1.5 g b.d.) for 4 weeks followed by a 4‐week washout phase. Molecular profiling of stool bacterial communities and IBS symptoms were assessed before, during and after mesalazine treatment. Colonic mucosal biopsies were assessed for proteolytic activity. Qualitative and quantitative effects of mesalazine on stool microbiota, mucosal proteolytic activity and IBS symptoms were assessed.

Results  Faecal bacteria decreased by 46% on mesalazine treatment (P = 0.014), but returned to baseline during washout. Firmicutes and Bacteroidetes represented 95% of identified phylotypes, with a trend towards an increase in the proportion of Firmicutes at week 4 in symptomatic responders [median (IQR) 14% (49) increase] compared with nonresponders [median 5% (11) decrease, P = 0.088]. Rectosigmoid mucosal proteolytic activity did not change between baseline and treatment [median 23.2 (17.9) vs. 19.5 (46.7) mU activity/mg tissue, P = 0.433]. Eight of 12 (67%) patients responded favourably to mesalazine based on a global relief questionnaire, with significant decreases in days with discomfort and increases in bowel movement satisfaction.

Conclusions  Mesalazine treatment is associated with a decrease in faecal bacteria abundance and rebalancing of the major constituents of the microbiota. Further study of the bacteriological and anti‐inflammatory properties of mesalazine in IBS is warranted.