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Published on: September 27, 2018
A mycolyl transferase mutant of Rhodococcus equi lacking capsule integrity is fully virulent
Tobias Sydor1, Kristine von Bargen, Ulrike Becken
1Institute for Cell Biology and Bonner Forum Biomedizin, University of Bonn, Ulrich-Haberland-Str. 61a, 53121 Bonn, Germany.
Abstract:
Rhodococcus equi is a mucoid Gram-positive facultative intracellular pathogen which can cause severe bronchopneumonia in foals and AIDS patients. A polysaccharide capsule which gives R. equi a mucoid appearance has long been suspected to be a virulence factor. Here, we describe a transposome mutant in the gene fbpA of strain R. equi 103 causing absence of a capsular structure. FbpA is a chromosomal gene homologous to antigen 85 (Ag85) mycolyl chain transferase gene of Mycobacterium tuberculosis. The mutant multiplied normally in isolated macrophages, was able to establish the unusual R. equi-containing vacuole in macrophages, was cytotoxic for macrophages, and was virulent in a mouse model. Colonies had a dry appearance on nutrient agar and defective capsule structure. Surprisingly, fbpA mutants cured of the virulence-associated plasmid were found in a phagosome that was more alkaline than that of the corresponding wild-type bacteria, were more cytotoxic and even multiplied to some extent. This study suggests that the capsule is not an important virulence factor of R. equi and that it may even counteract virulence traits.
Insights
The polysaccharide capsule of Rhodococcus equi, previously thought to be a virulence factor, was found not to be essential for virulence. Capsule absence in Rhodococcus equi mutants did not impair infection, suggesting it may even reduce virulence.
Area of Science:
- Microbiology
- Pathogenesis
- Bacterial genetics
Background:
- Rhodococcus equi is a Gram-positive pathogen causing severe bronchopneumonia in foals and AIDS patients.
- A polysaccharide capsule is characteristic of R. equi and is hypothesized to be a virulence factor.
Purpose of the Study:
- To investigate the role of the polysaccharide capsule in R. equi virulence.
- To characterize a transposome mutant in the fbpA gene, which is involved in capsule formation.
Main Methods:
- Generation of a transposome mutant in the fbpA gene of R. equi strain 103.
- Assessment of mutant growth and intracellular survival in macrophages.
- Evaluation of virulence in a mouse model.
- Analysis of phagosome pH and cytotoxicity.
Main Results:
- The fbpA mutant exhibited a defective capsule structure and dry colony morphology.
- The mutant showed normal multiplication in macrophages, established R. equi-containing vacuoles, was cytotoxic, and remained virulent in mice.
- fbpA mutants lacking the virulence plasmid showed increased phagosome alkalinity, enhanced cytotoxicity, and some multiplication.
Conclusions:
- The capsule of R. equi is not an essential virulence factor.
- The capsule may potentially inhibit or counteract certain virulence traits of R. equi.
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