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Virulence of Staphylococcus aureus mutants altered in type 5 capsule production
1Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115.
Abstract:
Most clinical isolates of Staphylococcus aureus produce microcapsules (uronic acid-containing extracellular polysaccharides) that are detectable by serologic methods but are not visible by negative staining. Among the 11 reported serotypes, capsule types 5 and 8 comprise approximately 75% of all isolates. Transposon mutagenesis was performed on S. aureus to create mutants altered in capsule expression. Tn918 was introduced into the capsule type 5 strain Reynolds by filter mating, and a capsule-deficient transconjugate, JL236, was isolated. The wild-type strain was transformed with JL236 chromosomal DNA to confirm that transfer of the appropriate-size chromosomal fragment containing Tn918 generated a capsule-deficient transformant. Strain Reynolds was mutagenized with ethyl methanesulfonate to obtain a capsule-negative mutant (strain JL240). Capsular phenotypes were determined by colony immunoblots, antibody adsorption experiments, and transmission electron microscopy. The virulences of the parental and mutant strains in mice were compared. The 50% lethal doses for strains Reynolds, JL236, and JL240 were similar (10(8.59), 10(8.98), and 10(8.93) CFU, respectively). Animals injected intraperitoneally with either wild-type or mutant strains had comparable levels of bacteremia at 3 and 24 h after challenge. Quantitative cultures of blood and kidneys from animals challenged intravenously with sublethal doses of the S. aureus strains also showed no differences in bacterial clearance or renal abscess formation. These studies indicate that the type 5 S. aureus microcapsule does not promote bacterial virulence in the animal models tested.
Insights
The Staphylococcus aureus type 5 microcapsule, a uronic acid-containing polysaccharide, does not enhance bacterial virulence. Studies in mice showed capsule-deficient mutants had similar lethal doses and infection levels compared to wild-type strains.
Area of Science:
- Microbiology
- Immunology
- Infectious Diseases
Background:
- Most clinical Staphylococcus aureus isolates produce microcapsules, primarily capsule types 5 and 8.
- These microcapsules are uronic acid-containing extracellular polysaccharides.
- Capsule expression is a key characteristic of S. aureus pathogenesis.
Purpose of the Study:
- To investigate the role of the type 5 S. aureus microcapsule in bacterial virulence.
- To generate and characterize capsule-deficient mutants of S. aureus.
- To compare the virulence of wild-type and mutant strains in a murine model.
Main Methods:
- Transposon mutagenesis using Tn918 and ethyl methanesulfonate treatment to create capsule-deficient mutants.
- Capsular phenotype determination via colony immunoblots, antibody adsorption, and transmission electron microscopy.
- Virulence assessment in mice by determining 50% lethal doses and quantifying bacteremia, bacterial clearance, and renal abscess formation.
Main Results:
- Capsule-deficient mutants (JL236 and JL240) were successfully generated from the type 5 S. aureus strain Reynolds.
- The 50% lethal doses for wild-type and mutant strains were statistically similar.
- No significant differences were observed in bacteremia levels, bacterial clearance, or renal abscess formation between wild-type and mutant strains in mice.
Conclusions:
- The type 5 S. aureus microcapsule does not contribute to bacterial virulence in the tested murine models.
- These findings suggest that the microcapsule may not be a primary virulence factor for S. aureus.
- Further research is needed to elucidate the precise role of microcapsules in S. aureus infections.