The K129 capsular polysaccharide produced by Acinetobacter baumannii MAR 15–4076 has the same composition as K84 but differs in the linkage between units altering the overall branching topology

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Capsular polysaccharide (CPS) is a heteroglycan that coats the cell surface of most isolates of the important Gram-negative bacterial pathogen, Acinetobacter baumannii. Strain MAR 15–4076, a clinical isolate recovered in Russia in 2015, was found to carry the KL129 sequence at the CPS biosynthesis K locus. The CPS was isolated from the strain and studied by sugar analysis, Smith degradation, one- and two-dimensional 1 H and 13 C NMR spectroscopy. It was composed of branched pentasaccharide units that include a →3)-α- l -Rha p -(1 → 3)-α- l -Rha p -(1 → 3)-β- d -Glc p NAc-(1→ mainchain and α- d -Man p NAc-(1 → 3)- l -Rha p side branch. Though the pentasaccharide units are identical to those that make up the K84 CPS produced by A. baumannii LUH5540, the units are linked differently via the substitution of an alternate l -Rha p residue, resulting in a difference in the overall topology of the CPS. This was due to the replacement of the Wzy polymerase gene encoded at the K locus. • Structure of the Acinetobacter baumannii K129 CPS determined. • K129 and K84 CPS have the same units but differ in CPS branching patterns. • Difference between structures was explained by a replacement of the Wzy polymerase gene at the CPS biosynthesis K locus.