Abstract
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.
Highlights
Enterobacterial common antigen (ECA) is a common surface antigen present in Gram-negative bacteria belonging to the Enterobacteriaceae family [1]
Contrary to ECA linked to phosphatidylglycerol (ECAPG) and ECALPS presented on the cell surface, ECACYC is located in the periplasm and has been recently pointed out as an important factor maintaining the outer membrane permeability barrier [4]
Since repeating units of both polymers are assembled on the same lipid carrier—undecaprenyl pyrophosphate (Und-PP)—and undergone similar processing [25], key features of ECALPS structures are predicted partially on the basis of biosynthesis pathway analyses: (i) ECALPS can only be observed in strains incapable of producing the O-specific polysaccharide (O-PS) [3,25]; (ii) ECA is ligated to the core OS in the position used to be occupied by O-PS, and (iii) an inverted anomeric configuration of the D-GlcpNAc residue in the first ECA repeating unit linked to the core OS has to be observed, whereas an α-configuration is a characteristic for polymeric chain (Figure 1) [3]
Summary
Enterobacterial common antigen (ECA) is a common surface antigen present in Gram-negative bacteria belonging to the Enterobacteriaceae family [1]. LPS is the main surface antigen of Gram-negative bacteria that typically comprises of three structural components: lipid A, core oligosaccharide (OS), and the O-specific polysaccharide (O-PS; O-antigen determining O serotype) [3]. ECAPG represents a major form of ECA and, together with LPS, is located on the cell surface, contributing to antigenicity, outer membrane integrity, and permeability, and to viability and virulence of bacteria. Contrary to ECAPG and ECALPS presented on the cell surface, ECACYC is located in the periplasm and has been recently pointed out as an important factor maintaining the outer membrane permeability barrier [4].
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