Macrophage toxicity and complement sensitivity of virulent and avirulent strains of Legionella pneumophila
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1Department of Microbiology, College of Medicine, University of Iowa, Iowa City 52242.
Abstract:
Virulent strains of Legionella pneumophila that are preincubated with specific antiserum are cytotoxic for alveolar macrophages when added at a multiplicity of infection of 100 L. pneumophila per macrophage. Under the same conditions, avirulent strains exhibit little cytotoxic activity. Virulent strains are also resistant to the bactericidal activity of normal human serum and fail to bind complement components on the cell surface. In contrast, avirulent strains are serum-sensitive and both C3 and C9 are deposited on the surface. Results of experiments suggest that the shift from virulence to avirulence of L. pneumophila may be the result of the selection of a small number of avirulent cells present in the population of a virulent culture.
Insights
Virulent Legionella pneumophila strains are cytotoxic to macrophages and resist serum killing. Avirulent strains are serum-sensitive, suggesting virulence shifts may arise from selecting rare avirulent cells within a virulent population.
Area of Science:
- Microbiology
- Immunology
Background:
- Legionella pneumophila is a bacterium that causes Legionnaires' disease.
- Understanding the mechanisms of virulence and avirulence is crucial for disease control.
Purpose of the Study:
- To investigate the differences in cytotoxic and serum resistance between virulent and avirulent Legionella pneumophila strains.
- To explore the potential mechanisms underlying the shift from virulence to avirulence.
Main Methods:
- Comparing the cytotoxicity of virulent and avirulent L. pneumophila strains against alveolar macrophages.
- Assessing the bactericidal activity of normal human serum on both strains.
- Analyzing the deposition of complement components (C3, C9) on bacterial surfaces.
Main Results:
- Virulent L. pneumophila strains, pre-incubated with antiserum, were cytotoxic to macrophages, unlike avirulent strains.
- Virulent strains resisted normal human serum bactericidal activity and did not bind complement components.
- Avirulent strains were serum-sensitive, with C3 and C9 deposited on their surfaces.
Conclusions:
- Differences in serum sensitivity and complement binding correlate with L. pneumophila virulence.
- The shift from virulence to avirulence may result from the selection of pre-existing avirulent variants within a virulent population.