title: Selective pressures and lipopolysaccharide subunits as determinants of resistance of clinical isolates of gram-negative bacilli to human serum.
abstract: Differences in molecular composition of lipopolysaccharides (LPS) between serum-sensitive (S) clinical isolates of Escherichia coli and serum-resistant (R) clones derived by serial passage in serum were demonstrated to determine sensitivity or resistance to killing by normal human serum (NHS). LPS from R clones had a greater proportion of higher-molecular-weight, more highly O-antigen-substituted subunits than LPS from their serum S parents. Utilization of a liposomal model with inserted LPS simulating bacterial cell walls established LPS as the site of serum bactericidal action. Liposomes containing S LPS were lysed, while liposomes containing R LPS were unaffected by NHS. R and S LPS were fractionated into higher (F1)- and lower (F2)-molecular-weight fractions. Liposomes containing R LPS or the F1 fraction of S and R LPS were not lysed by serum. Liposomes containing the F2 fraction of S or R LPS were lysed by serum analogous to that observed with liposomes containing intact S LPS. These findings establish LPS to be one site of serum bactericidal activity and demonstrate that the higher-molecular-weight, highly O-antigen-substituted LPS subunits mediate resistance to killing by NHS.
mesh_terms: D001433:Bacteriolysis; D001770:Blood Bactericidal Activity; D002850:Chromatography, Gel; D004591:Electrophoresis, Polyacrylamide Gel; D006090:Gram-Negative Bacteria / Q000276:immunology*; D006801:Humans; D008070:Lipopolysaccharides / Q000032:analysis / Q000502:physiology*; D008081:Liposomes / Q000008:administration & dosage; D008970:Molecular Weight; D011312:Pressure
chemical_list: D008070:Lipopolysaccharides; D008081:Liposomes