Enterobacterial common antigen (ECA) is a conserved surface antigen characteristic for Enterobacteriaceae. It is consisting of trisaccharide repeating unit, →3)-α-d-Fucp4NAc-(1→4)-β-d-ManpNAcA-(1→4)-α-d-GlcpNAc-(1→, where prevailing forms include ECA linked to phosphatidylglycerol (ECAPG) and cyclic ECA (ECACYC). Lipopolysaccharide (LPS)-associated form (ECALPS) has been proved to date only for rough Shigella sonnei phase II. Depending on the structure organization, ECA constitutes surface antigen (ECAPG and ECALPS) or maintains the outer membrane permeability barrier (ECACYC). The existence of LPS was hypothesized in the 1960–80s on the basis of serological observations. Only a few Escherichia coli strains (i.e., R1, R2, R3, R4, and K-12) have led to the generation of anti-ECA antibodies upon immunization, excluding ECAPG as an immunogen and conjecturing ECALPS as the only immunogenic form. Here, we presented a structural survey of ECALPS in E. coli R1, R2, R3, and R4 to correlate previous serological observations with the presence of ECALPS. The low yields of ECALPS were identified in the R1, R2, and R4 strains, where ECA occupied outer core residues of LPS that used to be substituted by O-specific polysaccharide in the case of smooth LPS. Previously published observations and hypotheses regarding the immunogenicity and biosynthesis of ECALPS were discussed and correlated with presented herein structural data.
The gut microbiota guide the development
of the host immune system
by setting a systemic threshold for immune activation. Lipopolysaccharides
(LPSs) from gut bacteria are able to trigger systemic and local proinflammatory
and immunomodulatory responses, and this capability strongly relies
on their fine structures. Up to now, only a few LPS structures from
gut commensals have been elucidated; therefore, the molecular motifs
that may be important for LPS–mammalian cell interactions at
the gut level are still obscure. Here, we report on the full structure
of the LPS isolated from one of the prominent species of the genus
Bacteroides
,
Bacteroides vulgatus
. The
LPS turned out to consist of a particular chemical structure based
on hypoacylated and
mono
-phosphorylated lipid A and
with a galactofuranose-containing core oligosaccharide and an O-antigen
built up of mannose and rhamnose. The evaluation of the immunological
properties of this LPS on human
in vitro
models revealed
a very interesting capability to produce anti-inflammatory cytokines
and to induce a synergistic action of MD-2/TLR4- and TLR2-mediated
signaling pathways.
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