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Interaction of complement with serum-sensitive and serum-resistant strains of Pseudomonas aeruginosa
Abstract:
The interaction of complement with the following two strains of Pseudomonas aeruginosa was examined: 144M, a mucoid, serum-sensitive strain bearing short lipopolysaccharide O chains, and 144M-SR, a mucoid, serum-resistant strain bearing long lipopolysaccharide O chains isolated by repeated passage of 144M in increasing concentrations of pooled normal human serum (PNHS). While significant killing of 144M occurred in 5 to 40% PNHS, no killing of 144M-SR was observed. Both strains activated complement, especially 144M-SR which consumed 88.7, 96.4, and 100% of the available complement 3 (C3), C5, and C9, respectively, in 10% PNHS during a 60-min incubation at 37 degrees C. Although it activated more C3 than did 144M (54.9% consumption), 144M-SR bound only half as much C3 as 144M. Similarly, although 144M-SR activated more C9 than did 144M (50.0% consumption in 60 min), there was considerably less C9 attached to 144M-SR (2,990 molecules of C9 per bacterium) than to 144M (13,700 molecules per bacterium) after 60 min of incubation. Furthermore, only 162 molecules of the C9 bound to 144M-SR remained bound after treatment with 0.1% trypsin, while 5,692 molecules of the C9 bound to 144M remained bound under similar conditions. These results show that the serum resistance of 144M-SR does not represent a failure to activate complement efficiently, but instead reflects failure of the assembled terminal complement complex C5b-9 to insert stably into the outer membrane of this strain.
Insights
Pseudomonas aeruginosa strain 144M-SR resists complement killing due to unstable C5b-9 complex insertion, not failed complement activation. This serum resistance is linked to lipopolysaccharide O chain length impacting complement interactions.
Area of Science:
- Immunology
- Microbiology
- Bacterial Pathogenesis
Background:
- Pseudomonas aeruginosa is an opportunistic pathogen.
- Complement system is crucial for innate immunity against bacterial infections.
- Serum resistance in bacteria can be mediated by lipopolysaccharide (LPS) structure.
Purpose of the Study:
- To investigate the interaction between complement and two Pseudomonas aeruginosa strains with differing serum resistance.
- To elucidate the mechanism underlying serum resistance in Pseudomonas aeruginosa 144M-SR.
Main Methods:
- Comparative analysis of complement activation and killing assays using serum-sensitive (144M) and serum-resistant (144M-SR) Pseudomonas aeruginosa strains.
- Quantification of complement component consumption (C3, C5, C9) and binding to bacterial surfaces.
- Assessment of terminal complement complex (C5b-9) stability on bacterial membranes after trypsin treatment.
Main Results:
- Pseudomonas aeruginosa 144M-SR exhibited significant resistance to complement-mediated killing, unlike the sensitive strain 144M.
- Both strains activated complement, with 144M-SR consuming higher percentages of C3, C5, and C9.
- Despite higher activation, 144M-SR bound less C3 and C9, and the bound C9 was less stable, indicating impaired C5b-9 insertion into its outer membrane.
Conclusions:
- Serum resistance of Pseudomonas aeruginosa 144M-SR is not due to inefficient complement activation.
- The primary mechanism is the failure of the terminal complement complex (C5b-9) to stably insert into the outer membrane of the resistant strain.
- Longer lipopolysaccharide O chains in 144M-SR likely contribute to this altered complement complex stability and serum resistance.