Pilus operon evolution in Streptococcus pneumoniae is driven by positive selection and recombination

Area of Science:

• Microbiology
• Evolutionary Biology
• Bacterial Pathogenesis

Background:

• Bacterial organelles evolve under selective pressures from host immune responses.
• The rlrA islet in Streptococcus pneumoniae encodes a pilus structure crucial for adherence and infection.
• Understanding these evolutionary forces is key to combating bacterial pathogens.

Purpose of the Study:

• To investigate the evolutionary dynamics of the rlrA islet in Streptococcus pneumoniae.
• To identify selective pressures acting on genes within the rlrA islet.
• To characterize recombination patterns in this important genomic region.

Main Methods:

• Sequencing of the rlrA islet from 44 clinical isolates of Streptococcus pneumoniae.
• Analysis of positive selection acting on specific genes (rrgA, rrgB, rrgC, srtB, srtC, srtD).
• Identification and analysis of homologous recombination events and hotspots.

Main Results:

• Genes encoding structural pilus components (rrgA, rrgB) are under positive selection.
• Genes for less exposed components (rrgC) and assembly enzymes (srtB, srtC, srtD) show no positive selection.
• Significant mosaicism and recombination hotspots were identified in the rlrA islet.

Conclusions:

• Genes directly interacting with the host immune system (surface-exposed) exhibit positive selection and diversity.
• Genes not directly exposed on the bacterial surface are more conserved due to recombination.
• This suggests a balance between immune evasion and functional conservation in bacterial evolution.