Lipopolysaccharide-Linked Enterobacterial Common Antigen (ECA(LPS)) Occurs in Rough Strains of Escherichia coli R1, R2, and R4

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 (ECA(PG)) and cyclic ECA (ECA(CYC)). Lipopolysaccharide (LPS)-associated form (ECA(LPS)) has been proved to date only for rough Shigella sonnei phase II. Depending on the structure organization, ECA constitutes surface antigen (ECA(PG) and ECA(LPS)) or maintains the outer membrane permeability barrier (ECA(CYC)). 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 ECA(PG) as an immunogen and conjecturing ECA(LPS) as the only immunogenic form. Here, we presented a structural survey of ECA(LPS) in E. coli R1, R2, R3, and R4 to correlate previous serological observations with the presence of ECA(LPS). The low yields of ECA(LPS) 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 ECA(LPS) were discussed and correlated with presented herein structural data.