Pseudomonas aeruginosa is a leading cause of urinary tract, pulmonary, and wound infections and is becoming increasingly resistant to antibiotics. Here, we report the iodonium- and gold(I)-promoted bimodal glycosylation of glycosyl (Z)-ynenoates for highly β-selective promoter-controlled synthesis and antigenic evaluation of a series of 1,2-cis-β-linked mannuronic acid alginate glycans of P. aeruginosa up to a 24-mer, which represents the longest polymannuronic acid synthesized to date. By screening the six synthetic mannuronic acid alginate glycans with the mouse serum antibodies, we identified the mannuronic acid tetrasaccharide as the optimal antigen epitope for the development of vaccines against P. aeruginosa.