5,7-Diamino-3,5,7,9-tetradeoxynon-2-ulosonic Acids in the Capsular Polysaccharides of Acinetobacter Baumannii

The polysaccharide capsule that surrounds the bacterial cell plays an important role in pathogenesis of infections caused by the opportunistic pathogen Acinetobacter baumannii by providing protection from external factors. Structures of the capsular polysaccharide (CPS) produced by individual A. baumannii isolates and the corresponding CPS biosynthesis gene clusters can be highly diverse but many from them are related. A large proportion of the A. baumannii CPS types contain an isomer of 5,7-diamino-3,5,7,9-tetradeoxynon-2-ulosonic acid. Three of these isomers, namely acinetaminic acid (L-glycero-L-altro isomer), 8-epiacinetaminic acid (D-glycero-L-altro isomer), and 8-epipseudaminic acid (D-glycero-L-manno isomer), have not been found so far in naturally-occurring carbohydrates from other species. In the CPSs of A. baumannii, the higher monosaccharides of this class carry N-acyl substituents at positions 5 and 7, and in some CPSs, the N-acetyl and N-(3-hydroxybutanoyl) groups are both present. The present review addresses the structures and genetics of biosynthesis of the CPSs of A. baumannii that contain di-N-acyl derivatives of 5,7-diamino-3,5,7,9-tetradeoxynon-2-ulosonic acids.