Evidence for rare capsular switching in Streptococcus agalactiae

Area of Science:

• Microbiology
• Genetics
• Immunology

Background:

• The polysaccharide capsule of Streptococcus agalactiae (group B Streptococcus [GBS]) is a key antigenic determinant.
• Previous studies suggested frequent capsular switching in GBS, despite its lack of natural transformability.

Purpose of the Study:

• To identify and characterize potential capsular switching events in GBS.
• To investigate the frequency and mechanisms of capsular switching.
• To assess the implications for GBS vaccine strategies.

Main Methods:

• Pulsed-field gel electrophoresis (PFGE) for genomic profiling.
• Multilocus sequence typing (MLST) for genetic relatedness.
• Surface protein and pilus gene profiling.
• Phenotypic and genotypic analyses for capsular switching.

Main Results:

• Capsular switching via horizontal gene transfer is less frequent in GBS than previously estimated.
• Serotyping errors, due to interpretation issues with phenotypic methods, likely contribute to overestimating switching events.
• Identified transformants acquired the entire capsular locus, not just serotype-specific genes.
• Clear evidence for in vivo capsular transformation in S. agalactiae was demonstrated.

Conclusions:

• The frequency of capsular switching in GBS may be overestimated, with serotyping errors being a significant factor.
• Horizontal gene transfer leading to capsular transformation does occur in vivo in S. agalactiae.
• Understanding capsular transformation is critical for developing effective GBS vaccination strategies.