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Klebsiella pneumoniae  is a Gram-negative, ubiquitous bacterium capable of causing severe nosocomial infections in individuals with impaired immune system. Emerging multi-drug resistant strains of this species and particularly carbapenem-resistant strains pose an urgent threat to public health. The lipopolysaccharide (LPS) O-antigen is the main surface antigen. It contributes to the virulence of this species and determines the O-serotype of  K. pneumoniae  isolates. Among the nine main O-serotypes of  K. pneumoniae , O1-and O2-type pathogens are causative agents of over 50% of all infections. Serotype O1, the most common O-serotype, expresses complex LPS consisting of  d -galactan-I (a polymer built of → 3)-β- d -Gal f -(1 → 3)-α- d -Gal p -(1 → repeating units) capped by  d -galactan-II (built of [ → 3)-α- d -Gal p -(1 → 3)-β- d -Gal p -(1 →] repeating units). Galactan-I is present as the sole polymer in O2 serotype. Recently, in case of serotype O2, conversion of galactan-I to galactan-III (→ 3)-β- d -Gal f -(1 → 3)-[α- d -Gal p -(1 → 4)]-α- d -Gal p -(1 →) was reported. Substitution of → 3)-α- d -Gal p  by a branching terminal α- d -Gal p  was dependent on the presence of the  gmlABC  operon and had a major impact on the antigenicity of the galactan polymer. Genetic analysis indicated that 40% of the O1 clinical isolates also carry the  gmlABC  locus; therefore we aimed to characterize the corresponding phenotype of LPS O-antigens. The presence of galactan-III among O1 strains was proven using galactan-III-specific monoclonal antibodies and confirmed by structural analyses performed using sugar and methylation analysis as well as classical and high-resolution magic angle spinning NMR spectroscopy. By using an isogenic mutant pair, we demonstrated that galactan-III expression was dependent on the presence of glycosyltransferases encoded by  gmlABC , as was shown previously for the O2 serotype. Furthermore, the galactan-II structures in O1 gml + strains remained unaffected corroborating no functional interactions between the biosynthesis of galactan-III and galactan-II polymers.

Identification of d-Galactan-III As Part of the Lipopolysaccharide of Klebsiella pneumoniae Serotype O1