Protective role of smooth lipopolysaccharide in the serum bactericidal reaction.

The mechanism by which smooth lipopolysaccharide (LPS) protects Salmonella typhimurium C5 from the lethal action of specific antibody and complement has been investigated. Tris(hydroxymethyl)aminomethane (Tris) and ethylenediaminetetraacetate (EDTA), which sensitize S. typhimurium C5 to the bactericidal action of immune serum, were shown to cause appreciable structural modification of the smooth LPS associated with these cells. Mg(2+) was shown to reverse the sensitization of C5 by Tris, but this divalent cation was unable to reverse the sensitizing effect of Tris plus EDTA, which, unlike Tris, caused relatively large amounts of O-antigenic components to be released from the C5 cell wall. The possibility that smooth LPS blocks the interaction of activated complement components with receptors on the cell wall is discussed.