  The colonic mucus layer is a dynamic and complex structure formed by secreted
and transmembrane mucins, which are high-molecular-weight and heavily
glycosylated proteins. Colonic mucus consists of a loose outer layer and a
dense epithelium-attached layer. The outer layer is inhabited by various
representatives of the human gut microbiota (HGM). Glycans of the colonic mucus
can be used by the HGM as a source of carbon and energy when dietary fibers are
not sufficiently available. Here, we analyzed 397 individual HGM genomes to
identify pathways for the cleavage of host-synthetized mucin glycans to
monosaccharides as well as for the catabolism of the derived monosaccharides.
Our key results are as follows: (i) Genes for the cleavage of mucin glycans
were found in 86% of the analyzed genomes, whereas genes for the catabolism of
derived monosaccharides were found in 89% of the analyzed genomes. (ii)
Comparative genomic analysis identified four alternative forms of the
monosaccharide-catabolizing enzymes and four alternative forms of
monosaccharide transporters. (iii) Eighty-five percent of the analyzed genomes
may be involved in exchange pathways for the monosaccharides derived from
cleaved mucin glycans. (iv) The analyzed genomes demonstrated different
abilities to degrade known mucin glycans. Generally, the ability to degrade at
least one type of mucin glycan was predicted for 81% of the analyzed genomes.
(v) Eighty-two percent of the analyzed genomes can form mutualistic pairs that
are able to degrade mucin glycans and are not degradable by any of the paired
organisms alone. Taken together, these findings provide further insight into
the inter-microbial communications of the HGM as well as into host-HGM
interactions.
