Unique lipid anchor attaches Vi antigen capsule to the surface ofSalmonella entericaserovar Typhi

Abstract

Polysaccharide capsules are surface structures that are critical for the virulence of many Gram-negative pathogenic bacteria.Salmonella entericaserovar Typhi is the etiological agent of typhoid fever. It produces a capsular polysaccharide known as “Vi antigen,” which is composed of nonstoichiometricallyO-acetylated α-1,4-linkedN-acetylgalactosaminuronic acid residues. This glycan is a component of currently available vaccines. The genetic locus for Vi antigen production is also present in soil bacteria belonging to the genusAchromobacter. Vi antigen assembly follows a widespread general strategy with a characteristic glycan export step involving an ATP-binding cassette transporter. However, Vi antigen producers lack the enzymes that build the conserved terminal glycolipid characterizing other capsules using this method.Achromobacterspecies possess a Vi antigen-specific depolymerase enzyme missing inS.entericaTyphi, and we exploited this enzyme to isolate acylated Vi antigen termini. Mass spectrometry analysis revealed a reducing terminalN-acetylhexosamine residue modified with two β-hydroxyl acyl chains. This terminal structure resembles one half of lipid A, the hydrophobic portion of bacterial lipopolysaccharides. The VexE protein encoded in the Vi antigen biosynthesis locus shares similarity with LpxL, an acyltransferase from lipid A biosynthesis. In the absence of VexE, Vi antigen is produced, but its physical properties are altered, its export is impaired, and a Vi capsule structure is not assembled on the cell surface. The structure of the lipidated terminus dictates a unique assembly mechanism and has potential implications in pathogenesis and vaccine production.