Pseudaminic acid (Pse, 5,7-diacetamido-3,5,7,9-tetradeoxy-L-glycero-L-manno-nonulosonic acid) is a bacterial nine-carbon nonulosonic acid structurally related to sialic acid (Neu5Ac) but found exclusively in prokaryotes, where it caps surface glycans, flagellins, and lipopolysaccharides of Gram-negative pathogens such as Campylobacter, Helicobacter, Pseudomonas, and Acinetobacter. As a non-mammalian sialic acid analog, pseudaminic acid is a valuable probe for studying bacterial glycobiology, virulence-associated glycosylation, host-pathogen interactions, and as a substrate or target for pseudaminic acid biosynthetic and antibody/lectin recognition studies.