Elucidation of the full O-polysaccharide structure and identification of the core type of the lipopolysaccharide of Providencia alcalifaciens O9

Opportunistic human pathogens of the genus Providencia from the family Enterobacteriaceae are serotyped by their O-antigens, which represent the O-polysaccharide chains of the lipopolysaccharides (LPSs) on the cell surface. In this work, the O-polysaccharide of Providencia alcalifaciens O9 was obtained by mild acid degradation of a long-chain S-form LPS. The structure of the hexasaccharide repeat (O-unit) of the O-polysaccharide containing one d-Gal, two d-Glc, and three d-GalNAc residues was established by sugar and methylation analyses along with one- and two-dimensional 1H and 13C NMR spectroscopy. Another degradation product was derived from a short-chain SR-form LPS and found to consist of a core oligosaccharide bearing one O-unit. Its studies by NMR spectroscopy and electrospray ionization mass spectrometry enabled identification of one of the GalNAc residues as the first monosaccharide of the O-unit, whose glycosidic linkage links the O-units to each other and the first O-unit to the core. The core is distinguished by the occurrence of two glycoforms differing in the nature of a lateral monosaccharide, which is either d-Glc or d-GlcNAc. Although composed of common monosaccharides, the O-polysaccharide of P. alcalifaciens O9 has a unique structure among bacterial polysaccharides, whereas the oligosaccharide region belongs to one of several core types recognized in the LPSs of Providencia.

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