Role of Intestinal Microbiota in Ulcerative Colitis

2. The colonic environment
Theoretical part
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2. The colonic environment
The commensal bacteria who inhabit the colon are either located in the lumen or in the mucus.
They are believed to play an important role in their specific environment, although it is not fully
understood what selects the mucosal and luminal bacterial communities. A recent review by Van
den Abbeele et al. (2011b) has suggested following hypothesis:
“Microorganisms who are not targeted by host defense molecules upon colonization of the mucus
layer, are able to reside in the mucosa associated microbial community, and commensals who are
preferentially targeted by host defense molecules in the mucus layer, are restricted to the lumen”.
A description of the mucosal and luminal environment will be given in the following chapter and in
addition, the role of the two microbial communities and how they interact with the host will be
addressed.
2.1. The mucosal environment
The mucus layer covering the colonic epithelium results in a physical network that separates the
lumen intestinal residents and pathogens from the host. The mucus layer can respond dynamically
to infection, and is regulated by the underlying innate and adaptive immune system (McGuckin et
al., 2011). The colonic mucus constitutes a complex fluid, which is rich in gel‐forming mucins. The
mucins are large glycoproteins characterized by abundant and variable O‐linked glycans attached
to hydroxy amino acids clustered in PTS domains (high frequency of the amino acids proline,
threonine, and serine). Sulfate residues and O‐acetyl‐substituted sialic acid can be found on the
terminal oligosaccharides of the glycans (Lang et al., 2007;Johansson et al., 2011;McGuckin et al.,
2011). The glycans are an important energy source for mucosal bacteria and are used as adhesion
sites for bacteria (Hansson and Johansson, 2010c;Johansson et al., 2011). However, they also serve
a different role. They protect the protein core from proteases, preserving the integrity of the
mucin polymer and due to their complex structure, decrease the number of bacteria able to grow
on mucin (Moncada et al., 2003).
The thickness of the colonic mucus is 700 μm and consists of two layers. The thin inner layer is a
firmly adherent gel, which is physically difficult to dislodge and devoid of bacteria (Johansson et
al., 2008;Hansson and Johansson, 2010b). The thick outer layer, on the other hand, is a loosely
adherent gel, which is more easily dispersed, colonized and degraded by bacteria (Atuma et al.,