Structure of the O-polysaccharide of Providencia alcalifaciens O3 containing 3,6-dideoxy-3-formamido-d-glucose and d-galacturonamide

► Lipopolysaccharide from Providencia alcalifaciens O3 was degraded with mild acid. ► An O-polysaccharide, core and O-unit-bearing core oligosaccharides were obtained. ► The isolated compounds were studied by chemical methods, NMR spectroscopy, and ESI MS. ► A new structure of a pentasaccharide repeat of the O-polysaccharide was established. ► A peptidoglycan-like polysaccharide was found in the lipopolysaccharide preparation too.