Role of Intestinal Microbiota in Ulcerative Colitis

Theoretical part
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2. The colonic environment
2001;Johansson et al., 2008). The mucus gel also provides a matrix for the retention of anti‐
microbial molecules and secretory immunoglobulin A (IgA). The antimicrobial peptides are
produced by the colonic epithelial cells, and include among others β‐defensins and cathelicidin
LL37 (Hase et al., 2002;Tollin et al., 2003), and the secretory IgA is synthesized by plasma cells in
the lamina propria.
The commensal bacteria colonizing the outer mucus layer have evolved different microbial
characteristics, which contribute to a specific selected mucosal community. Firstly, the bacteria
have to carry adhesion molecules to bind to the mucus (Laparra and Sanz, 2009;Kankainen et al.,
2009;Johansson et al., 2011). Cell surface components that promote the adherence of commensal
bacteria to mucus include among others; fimbriae (Schell et al., 2002;Kline et al., 2009;Kankainen
et al., 2009), elongation factor Tu (EF‐TU) (Granato et al., 2004;Gilad et al., 2011), extracellular
mucus‐binding protein (Mub, found in Lactobacillus spp.) (Roos and Jonsson, 2002) and lectin‐like
mannosespecific adhesin (Msa, found in Lactobacillus spp.) (Pretzer et al., 2005). Secondly, the
bacteria have to have the ability to gain nutrients from the host‐derived mucins by synthesizing
mucin‐degrading enzymes. This often involves the combined action of bacterial proteases,
glycosidases, sialidases and sulphatases, due to the complex structure of mucin (Killer and
Marounek, 2011). A variety of bacteria are able to produce one or more of the mucin‐degrading
enzymes, which include Akkermansia muciniphila and species of Bifidobacterium, Bacteroides,
Prevotella, and Ruminococcus (Bayliss and Houston, 1984;Wright et al., 2000;Derrien et al.,
2004;Killer and Marounek, 2011). Thirdly, the bacteria have to be able to survive in the presence
of an oxygen gradient along the mucus layer, as oxygen is continuously released from the blood
(Van den Abbeele et al., 2011b). Finally, the bacteria have to be resistant to antimicrobial
peptides. Species of Bacteroides have shown to be resistant to several host defensins allowing
their colonization of the outer layer mucus (Nuding et al., 2009). Moreover, it has been suggested
that the glycosylation pattern in mucins is an important factor for host selection of a specific
colonic mucosal community (Hansson and Johansson, 2010a). Recent study has shown that the
glycosylation pattern is complex but relatively conserved in the human colon compared to other
regions in the GI tract, hence specific adhesion molecules and mucin‐degrading enzymes are
needed suggesting selective advantages of the colonic commensal mucosal community (Larsson et
al., 2009).