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Effects on virulence of mutations in a locus essential for hyaluronic acid capsule expression in group A streptococci
M R Wessels1, J B Goldberg, A E Moses
1Channing Laboratory, Brigham and Women's Hospital, Boston, Massachusetts 02115.
Abstract:
Mucoid or highly encapsulated strains of group A streptococci have been associated both with unusually severe infections and with acute rheumatic fever. Previously, we described an acapsular mutant, TX4, derived from a mucoid M-type 18 strain of a group A streptococcus by transposon mutagenesis (M. R. Wessels, A. E. Moses, J. B. Goldberg, and T. J. DiCesare, Proc. Natl. Acad. Sci. USA 88:8317-8321, 1991). We now report studies further characterizing strain TX4 as well as an additional acapsular mutant, TX72. Strain TX4 was found to contain a 9.5-kb deletion of chromosomal DNA adjacent to the site of transposon Tn916 insertion. Cloned chromosomal DNA from TX4 flanking the transposon insertion site was used as a probe to demonstrate the presence of homologous regions in 11 of 11 wild-type group A streptococcal strains of various M protein types. A second acapsular mutant, TX72, had a single transposon insertion and had no apparent deletion of chromosomal DNA. The Tn916 insertion in TX72 was mapped to the hasA locus (encoding hyaluronate synthase), which lies within the chromosomal region deleted in TX4. Strain TX72 was avirulent in mice and sensitive to phagocytic killing in vitro. Transduction of either the insertion-deletion mutation from TX4 or the simple insertion mutation from TX72 to a type 24 group A streptococcus strain also resulted in loss of capsule expression, demonstrating that a homologous region of the chromosome controls capsule expression in another serotype of group A streptococci. We conclude that the hyaluronic acid capsule plays an important role in virulence and that a region of the chromosome essential for capsular polysaccharide expression is conserved among diverse group A streptococcal strains.
Insights
The hyaluronic acid capsule of group A streptococci is crucial for virulence and protection against phagocytosis. This study identifies a conserved chromosomal region essential for capsule production across diverse strains.
Area of Science:
- Microbiology
- Molecular Biology
- Pathogenesis
Background:
- Mucoid or highly encapsulated strains of group A streptococci are linked to severe infections and acute rheumatic fever.
- Previous work described an acapsular mutant (TX4) of a mucoid M-type 18 strain.
Purpose of the Study:
- Further characterize strain TX4 and an additional acapsular mutant, TX72.
- Investigate the role of the hyaluronic acid capsule in group A streptococcal virulence.
- Identify conserved genetic elements responsible for capsule expression.
Main Methods:
- Transposon mutagenesis to create acapsular mutants (TX4, TX72).
- Characterization of chromosomal DNA deletions and transposon insertion sites.
- Southern blot analysis using cloned DNA as probes.
- Phagocytosis assays in vitro and virulence studies in mice.
- Transduction experiments to transfer mutations to other serotypes.
Main Results:
- Strain TX4 exhibited a 9.5-kb deletion near the transposon insertion site.
- Homologous DNA regions were found in all tested wild-type group A streptococcal strains.
- Acapsular mutant TX72, with a single transposon insertion in the hasA locus, was avirulent and phagocytosis-sensitive.
- Transduction of mutations led to capsule loss in another serotype, confirming conserved genetic control.
Conclusions:
- The hyaluronic acid capsule is essential for group A streptococcal virulence and resistance to phagocytosis.
- A conserved chromosomal region is critical for capsular polysaccharide expression in group A streptococci.
- Understanding capsule genetics can inform strategies against severe streptococcal infections.