Capacity To Utilize Raffinose Dictates Pneumococcal Disease Phenotype

Area of Science:

• Microbiology
• Genetics
• Pathogenesis

Background:

• Streptococcus pneumoniae asymptomatically colonizes the human nasopharynx.
• Pneumococci cause invasive diseases (pneumonia, meningitis) and prevalent infections (otitis media).
• Distinct pathogenic profiles suggest niche adaptation within pneumococcal lineages.

Purpose of the Study:

• To identify genetic determinants of disease tropism in Streptococcus pneumoniae.
• To investigate the role of carbohydrate metabolism in pneumococcal pathogenesis.
• To understand factors governing the switch from commensal to pathogen.

Main Methods:

• Genomic analysis of blood and ear isolates from different serotypes and sequence types (STs).
• Phenotypic characterization of bacterial growth and gene expression in relation to raffinose utilization.
• Allelic exchange mutagenesis to assess the role of specific single nucleotide polymorphisms (SNPs) in vivo.

Main Results:

• SNPs in raffinose uptake and utilization genes (rafR, rafK) were identified in ear isolates compared to blood isolates.
• Ear isolates showed defective growth and reduced expression of raffinose pathway genes.
• A specific SNP in rafR determined differential tropism for lungs, ear, and brain in a mouse model.

Conclusions:

• The capacity of Streptococcus pneumoniae to utilize raffinose dictates the nature and site of disease.
• Convergent evolution is observed, with unrelated pneumococcal lineages showing similar adaptations in raffinose metabolism.
• Differential carbohydrate metabolism is critical in pneumococcal pathogenesis, distinguishing localized from invasive disease.