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Legionella pneumophila replication vacuoles mature into acidic, endocytic organelles
S Sturgill-Koszycki1, M S Swanson
1Department of Microbiology and Immunology, University of Michigan, Ann Arbor, Michigan 48109, USA.
Abstract:
After ingestion by macrophages, Legionella pneumophila inhibits acidification and maturation of its phagosome. After a 6-10-h lag period, the bacteria replicate for 10-14 h until macrophage lysis releases dozens of progeny. To examine whether the growth phase of intracellular L. pneumophila determines the fate of its phagosome, interactions between the endosomal network and pathogen vacuoles were analyzed throughout the primary infection period. Surprisingly, as L. pneumophila replicated exponentially, a significant proportion of the vacuoles acquired lysosomal characteristics. By 18 h, 70% contained lysosomal-associated membrane protein 1 (LAMP-1) and 40% contained cathepsin D; 50% of the vacuoles could be labeled by endocytosis, and the pH of this population of vacuoles averaged 5.6. Moreover, L. pneumophila appeared to survive and replicate within lysosomal compartments: vacuoles harboring more than five bacteria also contained LAMP-1, inhibition of vacuole acidification and maturation by bafilomycin A1 inhibited bacterial replication, bacteria within endosomal vacuoles responded to a metabolic inducer by expressing a gfp reporter gene, and replicating bacteria obtained from macrophages, but not broth, were acid resistant. Understanding how L. pneumophila first evades and then exploits the endosomal pathway to replicate within macrophages may reveal the mechanisms governing phagosome maturation, a process also manipulated by Mycobacteria, Leishmania, and Coxiella.
Insights
Legionella pneumophila initially evades macrophage defenses but surprisingly replicates within acidic, lysosome-like compartments. This suggests a novel exploitation of host cell pathways for bacterial growth and survival.
Area of Science:
- Microbiology
- Cell Biology
- Infectious Diseases
Background:
- Legionella pneumophila (L. pneumophila) is a bacterium that infects macrophages.
- Intracellular L. pneumophila inhibits phagosome acidification and maturation.
- Bacterial replication occurs after a lag phase, leading to macrophage lysis.
Purpose of the Study:
- To investigate if the intracellular growth phase of L. pneumophila influences its phagosome's fate.
- To analyze the interaction between the host endosomal network and pathogen vacuoles during infection.
Main Methods:
- Tracking phagosome-pathogen interactions throughout the primary infection period.
- Analyzing vacuole characteristics, including lysosomal markers (LAMP-1, cathepsin D), endocytic labeling, and pH.
- Assessing bacterial replication and acid resistance in relation to vacuole properties and bafilomycin A1 treatment.
Main Results:
- Contrary to expectations, a significant proportion of L. pneumophila vacuoles acquired lysosomal characteristics during exponential bacterial replication.
- By 18 hours post-infection, vacuoles showed increased LAMP-1 and cathepsin D content, endocytic labeling, and acidic pH (average 5.6).
- L. pneumophila survived and replicated within these lysosomal compartments, with replication dependent on vacuole acidification and maturation.
Conclusions:
- Intracellular L. pneumophila actively exploits the host endosomal pathway, replicating within compartments that acquire lysosomal features.
- This finding challenges the notion that bacteria solely evade phagolysosome fusion and suggests a mechanism of active manipulation.
- Understanding this process may illuminate general mechanisms of phagosome maturation and host-pathogen interactions relevant to other intracellular pathogens.