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Legionella pneumophila inhibits acidification of its phagosome in human monocytes
Abstract:
We used quantitative fluorescence microscopy to measure the pH of phagosomes in human monocytes that contain virulent Legionella pneumophila, a bacterial pathogen that multiplies intracellularly in these phagocytes. The mean pH of phagosomes that contain live L. pneumophila was 6.1 in 14 experiments. In the same experiments, the mean pH of phagosomes containing dead L. pneumophila averaged 0.8 pH units lower than the mean pH of phagosomes containing live L. pneumophila, a difference that was highly significant (P less than 0.01 in all 14 experiments). In contrast, the mean pH of phagosomes initially containing live E. coli, which were then killed by monocytes, was the same as for phagosomes initially containing dead E. coli. The mean pH of L. pneumophila phagosomes in activated monocytes, which inhibit L. pneumophila intracellular multiplication, was the same as in nonactivated monocytes. To simultaneously measure the pH of different phagosomes within the same monocyte, we digitized and analyzed fluorescence images of monocytes that contained both live L. pneumophila and sheep erythrocytes. Within the same monocyte, live L. pneumophila phagosomes had a pH of approximately 6.1 and sheep erythrocyte phagosomes had a pH of approximately 5.0 or below. This study demonstrates that L. pneumophila is capable of modifying the pH of its phagocytic vacuole. This capability may be critical to the intracellular survival and multiplication of this and other intracellular pathogens.
Insights
Legionella pneumophila manipulates phagosome pH, creating a less acidic environment for survival. This bacterial adaptation is crucial for intracellular multiplication and pathogenesis.
Area of Science:
- Microbiology
- Cell Biology
- Pathogen-Host Interactions
Background:
- Intracellular bacterial pathogens like Legionella pneumophila reside within host phagocytes.
- Phagosome acidification is a key defense mechanism of host cells.
- The interaction between Legionella pneumophila and the phagosomal environment is critical for its survival.
Purpose of the Study:
- To investigate the pH dynamics of phagosomes containing virulent Legionella pneumophila within human monocytes.
- To determine if Legionella pneumophila actively modifies its phagosomal environment.
- To compare the phagosomal pH of live versus dead Legionella pneumophila and other bacteria.
Main Methods:
- Quantitative fluorescence microscopy was employed to measure phagosome pH.
- Human monocytes were infected with virulent Legionella pneumophila.
- Simultaneous pH measurements were performed on phagosomes containing Legionella pneumophila and sheep erythrocytes within the same monocyte.
Main Results:
- Phagosomes containing live Legionella pneumophila exhibited a mean pH of 6.1.
- Phagosomes with dead Legionella pneumophila were significantly more acidic (0.8 pH units lower) than those with live bacteria.
- In contrast, Escherichia coli phagosomes showed no significant pH difference between live and dead bacteria.
- Within the same monocyte, Legionella pneumophila phagosomes (pH ~6.1) were less acidic than sheep erythrocyte phagosomes (pH ≤5.0).
Conclusions:
- Legionella pneumophila actively modifies the pH of its phagocytic vacuole.
- This pH modification is crucial for the intracellular survival and multiplication of Legionella pneumophila.
- The ability to alter phagosomal pH may be a common strategy for intracellular pathogens.