Exclusion of high-molecular-weight maltosaccharides by lipopolysaccharide O-antigen of Escherichia coli and Salmonella typhimurium.

The barrier properties of lipopolysaccharide were studied by testing the influence of O-antigen on the binding of ligand to maltoporin in the outer membranes of Escherichia coli and Salmonella typhimurium. Maltoporin (LamB protein) of Escherichia coli K-12 was capable of interacting with macromolecular starch polysaccharides, as was previously shown by the binding of intact bacteria to fluorescein-labeled amylopectin or to starch-Sepharose columns. In contrast, strains with complete O-antigenic lipopolysaccharide showed reduced binding to these substrates. A similar result was obtained with Salmonella typhimurium LT2, which did not bind to starch unless rfa mutations removed noncore polysaccharide. The exclusion limit of the lipopolysaccharide permeability barrier to alpha-glucans was tested by measuring the maltoporin-dependent transport of maltose and its inhibition by maltodextrins of various sizes. Only amylopectin (molecular weight, greater than 25,000) was excluded in transport experiments, whereas maltodextrins with molecular weights of up to 2,000 were not excluded by the presence of an O-polysaccharide layer.