Dysbiosis of the Intestinal Microbiota in IBS

The human gastrointestinal (GI) microbiota is a rich and dynamic community inhabited by approximately 1014 bacteria, most of which have not yet been cultivated in the laboratory (Zoetendal et al, 2006). The GI microbiota has been suggested as one of the etiological factors in irritable bowel syndrome (IBS), with a putative role in the development and maintenance of IBS symptoms (for a review, see Bolino & Bercik, 2010). The worldwide prevalence of IBS is 10-20% among adults and adolescents, depending on the diagnostic criteria applied (Longstreth et al, 2006). Abdominal pain or discomfort, irregular bowel movements and constipation or diarrhoea are common symptoms of IBS. Symptoms outside the GI tract, such as fatigue, anxiety and depression, are also often encountered. At its worst, IBS can cause significant effects on patients’ well-being, but it is not known to predispose to any severe illnesses. Patients can be grouped into three subtypes according to bowel habits: diarrhoea-predominant (IBS-D), constipation-predominant (IBS-C) or mixed-subtype (IBSM). However, the symptom subtype of each patient may vary over time (Longstreth et al, 2006). Compared to non-IBS controls, subjects with IBS have been associated with a greater temporal instability of the GI microbiota and quantitative changes have been detected within several distinct bacterial groups or species-like phylotypes, which are defined based solely on sequence data (see Table 1 for references). In analyses covering the overall microbial community, IBS subjects have shown a tendency to cluster apart from the healthy control subjects (Ponnusamy et al, 2011; Rajilić-Stojanović, 2007). Moreover, the IBS symptom-subgroups IBS have been proposed to differ from each other according to the GI microbiota of subjects within these groups (Lyra et al, 2009; Malinen et al, 2005; RajilicStojanovic, 2007). The most distinctive symptom sub-type is IBS-D, which could also be a result of the impact of the diarrhoea on the microbial environment in the gut. In addition, comparatively low quantities of bifidobacteria, which are generally considered beneficial to health, have been detected in several IBS studies (Balsari et al, 1982; Enck et al, 2009; Kerckhoffs et al, 2009; Krogius-Kurikka et al, 2009; Si et al, 2004). This finding, though still preliminary, encourages development of probiotic and prebiotic therapies for IBS. On the other hand, elevated numbers of Proteobacteria and Firmicutes, including Ruminococcus – like phylotypes, Lactobacillus sp. and Veillonella sp., have been reported. Quantitative and qualitative microbial alterations in the GI tract of IBS subjects may have a functional role in the syndrome aetiology or merely reflect the status of the gut, but still have diagnostic or prognostic value in clinical practise and research (Kassinen, 2009;