The Clostridium leptum group of bacteria is one of the dominant groups of fecal bacteria in adult humans, constituting 16-25% of the fecal microbiota [1,2]. This group, also called Clostridial cluster IV, includes Faecalibacterium prausnitzii and certain species of Eubacterium andRuminococcus[3]. Members of this group synergize with other intestinal microbiota to ferment unabsorbed dietary carbohydrate, producing short chain fatty acids of which butyrate is the major energy source for the colonic epithelium and profoundly influences intestinal epithelial function. F. prausnitzii, the most abundant member of this group, is a major producer of butyrate through carbohydrate fermentation [4]. It has acquired special status because of its association with anti-inflammatory effects in the gastrointestinal tract [5]. Any alteration in the composition of the gut microbiota that changes the microbial balance is likely to have repercussions on human health.