Chemical Synthesis of <i>Pseudomonas aeruginosa</i> , <i>Staphylococcus aureus</i> , and <i>Acinetobacter baumannii</i> Capsular Polysaccharide Fragments as Leads for Cross‐Protection

Abstract Pseudomonas aeruginosa and Staphylococcus aureus are listed by the World Health Organization as high‐priority multidrug‐resistant (MDR) pathogens, whereas Acinetobacter baumannii is classified as the critical‐priority group. These bacteria cause life‐threatening infections such as severe bloodstream, nosocomial, urinary tract, and soft‐tissue infections. Their cell surfaces display complex and structurally distinct glycans absent in host cells, making them targets for glycoconjugate vaccine and diagnostic research. In this study, we report the chemical synthesis of mono‐ and oligosaccharide fragments derived from three ESKAPE pathogens, P. aeruginosa O11, S. aureus (CP5, CP8, and strain M), and A. baumannii (S34 and O5), as well as Plesiomonas shigelloides O1. Glycan microarray screening revealed three epitopes exhibiting strong cross‐reactive immunogenicity against P. aeruginosa , S. aureus , and A. baumannii , demonstrating that a trisaccharide represents the minimal epitope required to elicit cross‐protective immune responses. The key features of P. aeruginosa O11 trisaccharide synthesis involve efficient assembly of a 1,2‐ cis ‐linked l ‐FucNAc–linker motif, followed by regioselective glycosylation at O3 and subsequently at O2 of the d ‐Glc– l ‐FucNAc–linker disaccharide. The same strategy was applied for assembling its tetrasaccharide fragment. Additionally, β ‐mannosylation and 1,2‐ cis ‐ d ‐FucNAc linkage formations were optimized for the S. aureus CP8 fragment, establishing a versatile route toward bacterial glycans relevant for vaccine and diagnostic development.