Structure of the K58 capsular polysaccharide produced by Acinetobacter baumannii isolate MRSN 31468 includes Pse5Ac7Ac that is 4-O-acetylated by a phage-encoded acetyltransferase.

Capsular polysaccharide (CPS), a heteropolymeric carbohydrate structure present on the cell surface of most isolates of the bacterial pathogen Acinetobacter baumannii , is a major virulence determinant. Here, the CPS produced by A. baumannii MRSN 31468, which carries the KL58 CPS biosynthesis locus, was studied by sugar analysis, one- and two-dimensional 1H and 13C NMR spectroscopy. The structure was found to consist of a repeating tetrasaccharide K-unit that includes glucose (d-Glc p), galactose (d-Gal p), N -acetyl-galactosamine (d-Gal p NAc), and 5,7-diacetamido-3,5,7,9-tetradeoxy- l - glycero - l - manno -non-2-ulosonic acid (5,7-di- N -acetylpseudaminic acid; Pse5Ac7Ac). The CPS has a branched repeating unit with the disaccharide →3)-β-d-Glc-(1→3)-β-d-GalNAc-(1→ as the mainchain and O-6 of the Glc unit substituted with the disaccharide β-Pse5Ac7Ac-(2→6)-α-d-Gal, and Pse5Ac7Ac is partially acetylated at O-4. The presence of Pse5Ac7Ac in the K58 structure is consistent with the presence of psaA-F genes in KL58, which are responsible for Pse5Ac7Ac synthesis. 4-O-acetylation of Pse5Ac7Ac was traced to an acetyltransferase, Atr44 , which was found to be closely related to Atr29 that similarly decorates Pse5Ac7Ac with 4OAc in the K46-type CPS. Atr44 like Atr29 is encoded by a gene found in a prophage. The K58 CPS produced by MRSN 31468 did not include the 8-epimer of Pse5Ac7Ac (5,7-di- N -acetyl-8-epipseudaminic acid; 8ePse5Ac7Ac) found in the closely related CPS from BAL062 that also carries KL58. Hence, the gene(s) for conversion of Pse5Ac7Ac to 8ePse5Ac7Ac must lie elsewhere. [Display omitted] • Structure of the CPS from Acinetobacter baumannii isolate MRSN31468 was determined. • Tetrasaccharide repeat units include D-Glc, D-Gal, D-GalNAc and Pse5Ac7Ac. • Structure was found to be consistent with genetic content of KL58 locus in the genome of MRSN31468. [ABSTRACT FROM AUTHOR]