Role of lipopolysaccharide and complement in susceptibility of Klebsiella pneumoniae to nonimmune serum.

The role of lipopolysaccharide (LPS) in the susceptibility of Klebsiella pneumoniae to serum and the mechanism of complement activation by serum-susceptible (SerS) strains were investigated. The classical and alternative complement pathways are involved in serum killing of susceptible K. pneumoniae strains. The LPS composition seems to play a very important role in the serum bactericidal reaction, while capsular polysaccharide from this bacterium does not play any role. High-molecular-weight LPS from serum-resistant (Serr) K. pneumoniae strains was able to inhibit completely the serum bactericidal activity. LPS from SerS K. pneumoniae strains was not able to inhibit completely the serum bactericidal activity; low-molecular-weight LPS from Serr K. pneumoniae strains could not either. All these findings suggested that LPS composition, especially the O-antigen polysaccharide chains, contributes to the susceptibility of K. pneumoniae strains to complement-mediated serum bactericidal activity.