Characterization of the susceptibility of Pseudomonas aeruginosa to complement-mediated killing: role of antibodies to the rough lipopolysaccharide on serum-sensitive strains.

The mechanism of complement-mediated killing of seven serum-sensitive Pseudomonas aeruginosa strains was examined. All seven strains were sensitive to the bactericidal activity of 20% pooled normal human serum (PNHS) containing magnesium EGTA, which blocks the classical complement pathway (CCP), or 20% PNHS preheated to 50 degrees C for 20 min, which inactivates the alternative complement pathway, suggesting that either pathway was effective against these strains. However, for four of these strains, optimal killing required the function of both pathways. Preabsorption of PNHS with serum-sensitive strains dramatically reduced the killing activity of serum for the homologous strains when a concentration of 10% serum was used, implying a role for antibody in the activation of complement via the CCP. Affinity purification of antibodies to the rough lipopolysaccharide (LPS) on strain 144M resulted in a pool of antibodies which could restore all of the bactericidal activity and most of the C3 activation-deposition activity of serum which had been lost by preabsorption with 144M. Confirmation that the LPS was the target for these bactericidal antibodies was provided by demonstrating that exogenously added 144M LPS inhibited the killing activity of PNHS. These anti-144M LPS-specific antibodies were also bactericidal for the six other serum-sensitive strains examined, suggesting that all seven strains shared an antigenic determinant recognized by these anti-144M LPS-specific antibodies. Results from cross-absorption studies imply that there are bactericidal antibodies in PNHS directed to additional bacterial targets. These studies suggest that part of the bactericidal activity of PNHS is due to binding of antibodies to the rough LPS on serum-sensitive strains, initiating activation of the CCP, and that all seven strains examined shared this bactericidal antibody-binding site.