Structure of the neutral capsular polysaccharide of Acinetobacter baumannii NIPH146 that carries the KL37 capsule gene cluster

• A neutral capsular polysaccharide was isolated from Acinetobacter baumannii NIPH146.
• Structure of the polysaccharide was established by NMR and chemical approaches.
• Capsule biosynthesis gene cluster was analyzed and found to match the structure established.
• Glycosyltransferases encoded in the cluster were tentatively assigned to glycosidic linkages.

Capsular polysaccharide (CPS) was isolated from Acinetobacter baumannii NIPH146, and the following structure of branched pentasaccharide repeating unit was established by sugar analyses along with 1D and 2D NMR spectroscopy:In comparison to most other known capsular polysaccharides of A. baumannii, the CPS studied is neutral and lacks any specific monosaccharide component. The synthesis, assembly and export of this structure could be attributed to genes in a novel capsule biosynthesis gene cluster, designated KL37, which was found in the NIPH146 genome. The CPS of A. baumannii NIPH146 shares the α-d-Galp-(1→6)-β-d-Glcp-(1→3)-d-GalpNAc-(1→ trisaccharide fragment with the CPS units of several A. baumannii strains, including ATCC 17978 and LUH 5537 that carry the KL3 and KL22 gene clusters, respectively. KL37 contains two genes for glycosyltransferases that are related to two glycosyltransferase genes present in both KL3 and KL22, and the encoded proteins could be tentatively assigned to linkages between sugars in the CPS repeat.

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