O-acetyl groups shape the serological specificity and division of phosphodiester-containing Proteus mirabilis O48 O-antigens into subgroups.

Bacteria within the genus Proteus are serologically differentiated and classified so far to 86 O serogroups. The aim of the study was to determine the O serotype of serologically unique, related clinical isolates P. mirabilis Sr81 and Bm133, to chemically characterize the O antigen of a representative strain, and to analyse its structure importance in the LPS serospecificity. The chemical and structural analyses of the representative P. mirabilis Sr81 O antigen, including 1H, 13C and 31P NMR spectroscopy, and MALDI-TOF mass spectrometry, revealed that the O repeating unit is linear and contain four sugar components interlinked by a glycosidic phosphate. Its structure relates to P. mirabilis O48 polysaccharide, which additionally possesses an acetyl group linked to α-d-GlcpNAc. However, only slight serological cross reactions were observed in ELISA and Western blotting between Sr81/Bm133 antisera and the O48 LPSs, while the O48 antiserum did not recognize the Sr81 or Bm133 LPS epitopes, which emphasises the role of side O-acetyl groups in the O-polysaccharides specificity. We propose to include the studied strains in the newly created O48a subgroup which broaden the Proteus serological classification scheme. The results should be taken into account in serological analyses and works on a potential anti-Proteus vaccine construction.