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Salmonella stimulate macrophage macropinocytosis and persist within spacious phagosomes
C M Alpuche-Aranda1, E L Racoosin, J A Swanson
1Infectious Disease Unit, Massachusetts General Hospital, Boston 02114.
Abstract:
Light microscopic studies of phagocytosis showed that Salmonella typhimurium entered mouse macrophages enclosed in spacious phagosomes (SP). Viewed by time-lapse video microscopy, bone marrow-derived macrophages exposed to S. typhimurium displayed generalized plasma membrane ruffling and macropinocytosis. Phagosomes containing Salmonella were morphologically indistinguishable from macropinosomes. SP formation was observed after several methods of bacterial opsonization, although bacteria opsonized with specific IgG appeared initially in small phagosomes that later enlarged. In contrast to macropinosomes induced by growth factors, which shrink completely within 15 min, SP persisted in the cytoplasm, enlarging often by fusion with macropinosomes or other SP. A Salmonella strain containing a constitutive mutation in the phoP virulence regulatory locus (PhoPc) induced significantly fewer SP. Similar to Yersinia enterocolitica, PhoPc bacteria entered macrophages in close-fitting phagosomes, consistent with that expected for conventional receptor-mediated phagocytosis. These results suggest that formation of SP contributes to Salmonella survival and virulence.
Insights
Salmonella typhimurium invades macrophages using spacious phagosomes (SP), a process crucial for bacterial survival. A mutation in the phoP virulence gene reduced SP formation, suggesting SPs are key to Salmonella virulence.
Area of Science:
- Microbiology
- Cell Biology
- Immunology
Background:
- Salmonella typhimurium is a significant bacterial pathogen.
- Phagocytosis is a key cellular process for immune defense against bacteria.
- The mechanisms of Salmonella entry into host cells are not fully understood.
Purpose of the Study:
- To investigate the mechanism of Salmonella typhimurium entry into macrophages.
- To characterize the formation and properties of spacious phagosomes (SP) during Salmonella infection.
- To determine the role of the phoP virulence gene in SP formation and bacterial entry.
Main Methods:
- Light microscopy and time-lapse video microscopy of mouse bone marrow-derived macrophages.
- Observation of Salmonella typhimurium interactions with macrophages.
- Analysis of phagosome morphology and dynamics.
- Comparison of wild-type and phoP mutant Salmonella strains.
Main Results:
- Salmonella typhimurium entered macrophages within spacious phagosomes (SP), distinct from macropinosomes.
- SP formation was observed with various opsonization methods, with IgG-opsonized bacteria initially in small phagosomes.
- SP persisted and enlarged, unlike transient macropinosomes.
- A phoP mutant (PhoPc) induced significantly fewer SP and entered macrophages in close-fitting phagosomes, similar to conventional phagocytosis.
Conclusions:
- Spacious phagosome formation is a mechanism utilized by Salmonella typhimurium to enter macrophages.
- The phoP virulence gene is important for inducing SP formation.
- SP formation likely contributes to Salmonella survival and virulence within host cells.