Identification of a functional capsule locus in Streptococcus mitis.

Area of Science:

• Microbiology
• Bacterial Pathogenesis
• Oral Microbiology

Background:

• The polysaccharide capsule of Streptococcus pneumoniae is crucial for virulence.
• Streptococcus mitis is a common human commensal found in the oral cavity.
• Genomic analysis revealed putative capsule loci in some S. mitis strains.

Purpose of the Study:

• To investigate the function of the capsule locus in S. mitis.
• To characterize the S. mitis capsule-like structure and its impact on surface properties.
• To compare the S. mitis capsule's role with that in S. pneumoniae.

Main Methods:

• Construction of an isogenic S. mitis mutant (Δcps) lacking the capsule locus.
• Transmission electron microscopy and atomic force microscopy to visualize the capsule.
• Assays for hydrophobicity, surface charge, autoaggregation, and biofilm formation.

Main Results:

• A capsule-like structure was observed in the S. mitis type strain but absent/reduced in the Δcps mutant.
• The Δcps mutant showed increased hydrophobicity and autoaggregation.
• Biofilm formation increased up to six-fold in the Δcps mutant.

Conclusions:

• The S. mitis capsule locus is associated with a capsule-like structure.
• This capsule influences S. mitis surface properties, autoaggregation, and biofilm formation.
• The S. mitis capsule may confer similar surface attributes to the S. pneumoniae capsule.