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Characterization of a plasmid region involved in Bacillus anthracis toxin production and pathogenesis
J C Sirard1, C Guidi-Rontani, A Fouet
1Unité des Toxines et Pathogénie Bactériennes, URA 1858 du CNRS, Institut Pasteur, Paris, France.
Abstract:
The germination of spores within the host is the initial step of anthrax infection. We have shown, using immunofluorescence staining, confocal scanning laser microscopy and image cytometry analysis, that the alveolar macrophage is the primary site of B. anthracis germination in a murine inhalation infection model. B. anthracis germinated inside macrophages, in vesicles derived from the phagosomal compartment. We have demonstrated that the toxin genes and their trans-activator, AtxA, are expressed within the macrophages after germination. It was also shown that the pXO1 plasmid strongly enhanced capsule formation and that this influence is mediated by AtxA. This indicates the existence of a regulon where AtxA is the regulatory protein acting on genes located on different plasmids. We identified a tricistronic germination operon gerX located between the pag and atxA genes on the 40-kb toxin-encoding fragment of pXO1 . Analysis of a gerX null mutant indicated that gerX-encoded proteins are involved in the virulence of B. anthracis.
Insights
Bacillus anthracis spores germinate inside alveolar macrophages, initiating anthrax infection. The gerX operon and AtxA protein are crucial for B. anthracis virulence and toxin gene expression within host cells.
Area of Science:
- Microbiology
- Immunology
- Pathogenesis
Background:
- Anthrax infection begins with Bacillus anthracis spore germination within the host.
- Understanding the initial cellular events of B. anthracis infection is critical for developing effective countermeasures.
Purpose of the Study:
- To identify the primary host cell type involved in B. anthracis germination.
- To investigate the expression of virulence factors, including toxin genes and capsule formation, during early infection.
- To elucidate the role of the gerX operon in B. anthracis pathogenesis.
Main Methods:
- Immunofluorescence staining
- Confocal scanning laser microscopy
- Image cytometry analysis
- Construction and analysis of a gerX null mutant
Main Results:
- Alveolar macrophages are the primary site of B. anthracis germination in a murine inhalation model.
- B. anthracis germinates within macrophage-derived vesicles, expressing toxin genes and the trans-activator AtxA.
- The pXO1 plasmid, mediated by AtxA, significantly enhances capsule formation, suggesting a regulon.
- A novel tricistronic germination operon, gerX, was identified and shown to be involved in virulence.
Conclusions:
- Alveolar macrophages are key host cells for B. anthracis germination and early virulence factor expression.
- AtxA plays a central regulatory role in B. anthracis virulence, controlling toxin and capsule gene expression.
- The gerX operon is essential for B. anthracis virulence, highlighting a new target for therapeutic intervention.