Bacterial Glycans
Bacterial glycans are essential in the colonization of bacteria,
invasion of host tissues, and modulation of immune responses in the
gut104. Glycoconjugates produced by bacteria include
LPS, teichoic acids, glycoproteins, glycolipids, peptidoglycans, and
capsular polysaccharides233. Some species, such asPseudomonas aeruginosa and Neisseria meningitidis express
O-linked glycoproteins, mainly found in pilin and flagellin subunits,
and other species, such as Haemophilus influenzae express
N-linked glycoproteins234,235. L. plantarum , a
commensal bacterium, expresses an O-glycosylated protein, Acm2, as its
major autolysin236 . The glycosylation machinery
in C. jejuni mostly relies on oligosaccharyltransferase PglB,
which transfers oligosaccharides to proteins237.C. jejuni deficient in PglB have attenuated pathogenesis,
highlighting the microbe’s reliance on its glycosylation
system238.
Bacterial surface glycans can be recognized by immune
cells239. One example is when the glycans expressed by
bacteria are recognized by host lectins such as CLRs, galectins, and
Siglecs. This recognition may result in the engulfment of the microbe by
DCs, allowing for the processing and presentation of immunogenic
epitopes103,240,241. Zwitterionic polysaccharide (ZPS)
capsules in bacteria are highly studied immunomodulatory bacterial
glycans, which play many important roles in the regulation of intestinal
immune homeostasis242. For example, Polysaccharide A
(PSA) of B. fragilis , a ZPS, induces the production of IL-10
through APCs and thereby creates a tolerogenic environment to
colonize243. It can also skew the T-helper balance in
favor of Th1 and help maintain the Th1/Th2 balance in the
gut80. Bacteria can also mimic host glycan structures
(molecular mimicry)152,244. Bacteria like B.
fragilis can use the free fucose on their capsule and imitate the
fucosylated epithelial glycans245. This process also
aids in immune evasion of the bacteria152.
Apart from expressing glycans on their surface, gut bacteria also
produce enzymes that modulate glycan expression on immune
cells103. For example, immune evasion byStreptococcus pyogenes (Group A streptococcus), a pathogen that
causes diseases like acute rheumatic fever and post-streptococcal
glomerulonephritis, is attained by direct glycan modulation on
antibodies246,247. A targeted mass spectrometry study
has shown that this is achieved by an endoglycosidase, EndoS, that
cleaves the conserved N-glycan on IgG antibodies246.