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Identification of a Legionella pneumophila locus required for intracellular multiplication in human macrophages
A Marra1, S J Blander, M A Horwitz
1Department of Microbiology, College of Physicians and Surgeons, Columbia University, New York, NY 10032.
Abstract:
The legionnaires' disease bacterium, Legionella pneumophila, is a facultative intracellular parasite. Its interaction with phagocytes has characteristics in common with several other intracellular parasites. Critical aspects of L. pneumophila intracellular multiplication are evasion of lysosomal host cell defenses and the presence of a nutritionally appropriate environment. Following phagocytosis, wild-type L. pneumophila multiply within a specialized phagosome which does not fuse with secondary lysosomes. Mutants which have lost the ability to grow within phagocytes no longer cause disease in animals, indicating that the capacity to multiply intracellularly is important for pathogenesis. One such mutant, 25D, has been shown to be defective in inhibiting phagosome-lysosome fusion. This phagolysosomal environment is not conducive to Legionella growth. We report the isolation of a region of the L. pneumophila genome (icm, intracellular multiplication) which restores the capacity of 25D to multiply in human macrophages. The complemented mutants also regain the capacity to interfere with phagosome-lysosome fusion and to cause lethal pneumonia in guinea pigs.
Insights
Legionella pneumophila causes disease by multiplying inside host cells. A specific gene region (icm) was found to restore this intracellular multiplication ability in mutants, enabling them to cause lethal pneumonia.
Area of Science:
- Microbiology
- Pathogenesis
- Cell Biology
Background:
- Legionella pneumophila is a facultative intracellular parasite.
- Intracellular multiplication within host phagocytes is crucial for L. pneumophila pathogenesis.
- Successful intracellular growth requires evading lysosomal defenses and establishing a suitable environment.
Purpose of the Study:
- To identify genetic factors enabling Legionella pneumophila intracellular multiplication.
- To understand the mechanism of phagosome-lysosome fusion evasion by L. pneumophila.
- To restore the virulence of a mutant strain defective in intracellular growth.
Main Methods:
- Isolation and characterization of Legionella pneumophila mutants.
- Complementation of mutant strains using genomic DNA.
- Assessment of intracellular multiplication in human macrophages.
- Evaluation of virulence in a guinea pig pneumonia model.
Main Results:
- A specific genomic region, termed icm (intracellular multiplication), was identified.
- Complementation with the icm region restored intracellular multiplication in macrophages for mutant 25D.
- Complemented mutants regained the ability to inhibit phagosome-lysosome fusion.
- Complemented mutants caused lethal pneumonia in guinea pigs, similar to wild-type bacteria.
Conclusions:
- The icm region is essential for Legionella pneumophila intracellular multiplication and virulence.
- Interference with phagosome-lysosome fusion is a key mechanism for L. pneumophila pathogenesis.
- Restoration of icm function re-establishes the bacterium's ability to cause disease.