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Phase-shift of polysaccharide capsule expression in group B streptococci, type III
M Sellin1, S Håkansson, M Norgren
1Department of Clinical Bacteriology, Umeå University, Sweden.
Microbial Pathogenesis
|June 1, 1995
Summary
Group B Streptococcus (GBS) capsule expression can reversibly shift, impacting virulence. Frequencies of these phase-shifts vary significantly between strains, with pH influencing capsule production.
Area of Science:
- Microbiology
- Bacterial Pathogenesis
- Molecular Biology
Background:
- The polysaccharide capsule of Group B Streptococcus (GBS) is a key virulence factor.
- An inverse relationship exists between GBS buoyant density and capsular thickness.
Purpose of the Study:
- To investigate the frequencies of polysaccharide capsular phase-shifting in clinical GBS type III strains.
- To explore the reversibility and influencing factors of GBS capsular phase-shifts.
Main Methods:
- Assessed phase-shift frequencies using buoyant density measurements.
- Validated phenotypic shifts with flow cytometry.
- Evaluated the impact of culture conditions (temperature, oxygen, serum, pH) on capsule expression.
Main Results:
- Phase-shift frequencies from intermediate to high density (poor encapsulation) ranged from 1.2 x 10(-3) to 4.8 x 10(-6).
- Phase-shift frequencies from intermediate to low density (abundant encapsulation) ranged from 1.9 x 10(-4) to 1.1 x 10(-7).
- Shifts were reversible, with reversion frequencies up to 10(-1); significant strain-to-strain variation observed.
- Low pH (below 5.5) reduced capsule-bound polysaccharide, likely via phenotypic modification.
- IS861 insertion sequence was present in multiple copies but showed no correlation with encapsulation phenotype.
Conclusions:
- GBS type III strains exhibit significant and reversible capsular phase-shifting.
- Environmental factors like pH can modulate capsule expression phenotypically.
- The IS861 insertion sequence does not appear to directly regulate GBS capsule expression in the studied strains.