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

Background

The evolution of bacterial organelles involved in host-pathogen interactions is subject to intense and competing selective pressures due to the need to maintain function while escaping the host immune response. To characterize the interplay of these forces in an important pathogen, we sequenced the rlrA islet, a chromosomal region encoding for a pilus-like structure involved in adherence to lung epithelial cells in vitro and in colonization in a murine model of infection, in 44 clinical isolates of Streptococcus pneumoniae.

Results

We found that the rrgA and rrgB genes, encoding the main structural components of the pilus, are under the action of positive selection. In contrast, the rrgC gene, coding for a component present in low quantities in the assembled pilus, and the srtB, srtC and srtD genes, coding for three sortase enzymes essential for pilus assembly but probably not directly exposed to the host immune system, show no evidence of positive selection. We found several events of homologous recombination in the region containing these genes, identifying 4 major recombination hotspots. An analysis of the most recent recombination events shows a high level of mosaicism of the region coding for the rrgC, srtB, srtC and srtD genes.

Conclusions

In the rlrA islet, the genes coding for proteins directly exposed to the host immune response are under the action of positive selection, and exist in distinct forms in the population of circulating strains. The genes coding for proteins not directly exposed on the surface of the bacterial cell are more conserved probably due to the homogenizing effect of recombination.