Role of Intestinal Microbiota in Ulcerative Colitis

Theoretical part
4
1. The intestinal environment
Metagenomic studies have revealed that the majority of gut microbiota sequences belong to the
Bacteria, reflecting their predominance in the human adult gut (Eckburg et al., 2005;Qin et al.,
2010;Arumugam et al., 2011). Despite the complexity of the human intestinal ecosystem, the
majority of the bacteria are members of only a limited number of dominating bacterial phyla, such
as Bacteroidetes, Firmicutes, Actinobacteria, Proteobacteria, Verrucomicrobia, and Fusobacteria
with Firmicutes, and Bacteroidetes being the most abundant phyla (Figure 1)(Eckburg et al.,
2005;Tap et al., 2009;Arumugam et al., 2011). Within the Firmicutes phylum, 95% of the
phylogenetic types are members of the Clostridia class, and of these a substantial number are
related to butyrate‐producing bacteria, all of which fall within the clostridial clusters IV, XIVa, and
XVI (Eckburg et al., 2005;Tap et al., 2009). At genus level, Bacteroides has shown to be the most
abundant genus in the human gut microbiota of adults, followed by the genera Faecalibacterium,
Bifidobacterium, Lachnospiraceae, Roseburia, and Alistipes (Arumugam et al., 2011). Another
genus normally detected in the human gut is Lactobacillus, although only present in low levels
depending on age and individuals (Mueller et al., 2006;Frank et al., 2007).
Figure 1: The microbial diversity of the main phylotypes in the human intestinal microbiota (Turroni et al.,
2009).