Capsule type of Streptococcus pneumoniae determines growth phenotype

Area of Science:

• Microbiology
• Bacterial Pathogenesis
• Molecular Biology

Background:

• The polysaccharide capsule of Streptococcus pneumoniae is a key virulence factor, defining over 90 serotypes with varying carriage prevalence and invasiveness.
• Previous research suggested a link between serotype and in vitro growth, but the role of capsule production in this relationship remained unclear.

Purpose of the Study:

• To investigate if capsule production interferes with Streptococcus pneumoniae growth in vitro.
• To determine if this in vitro growth phenotype predicts in vivo carriage prevalence.
• To explore the relationship between capsule structure, energy expenditure for capsule production, and carriage prevalence.

Main Methods:

• Construction of 81 capsule switch mutants representing nine different Streptococcus pneumoniae serotypes.
• Assessment of bacterial growth (lag phase, optical density) in nutrient-restricted (MLM) and rich (BHI+FCS) media.
• Evaluation of nasopharyngeal colonization efficiency in mice.
• Analysis of capsule thickness via electron microscopy, FITC-dextran exclusion, and HPLC.
• Quantification of cpsA gene expression using RT-PCR and capsule production energy costs via H3-glucose incorporation.

Main Results:

• Bacterial growth phenotype and mouse nasopharyngeal colonization efficiency were dependent on and transferable with the capsule operon type.
• Capsule production interfered with growth in nutrient-limiting conditions, inversely correlating with serotype-specific carriage prevalence.
• Serotypes with higher carriage prevalence produced thicker capsules, but exhibited delayed growth in nutrient-limited media.
• Energy expenditure for capsule production relative to capsule amount was higher for low-prevalence serotypes.
• Differences in growth and capsule production between serotypes were not apparent in rich media.

Conclusions:

• Polysaccharide capsule production in Streptococcus pneumoniae interferes with growth in nutrient-limiting environments, likely due to energy competition with central metabolism.
• The in vitro growth phenotype accurately predicts serotype-specific nasopharyngeal carriage prevalence in vivo.
• A trade-off exists between capsule thickness, energy investment, growth rate, and in vivo prevalence for different Streptococcus pneumoniae serotypes.