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.