Competence in Streptococcus pneumoniae and Close Commensal Relatives: Mechanisms and Implications.

Gabriela Salvadori,Roger Junges,Donald A Morrison,Fernanda C Petersen

doi:10.3389/fcimb.2019.00094

Abstract

The mitis group of streptococci comprises species that are common colonizers of the naso-oral-pharyngeal tract of humans. Streptococcus pneumoniae and Streptococcus mitis are close relatives and share ~60–80% of orthologous genes, but still present striking differences in pathogenic potential toward the human host. S. mitis has long been recognized as a reservoir of antibiotic resistance genes for S. pneumoniae, as well as a source for capsule polysaccharide variation, leading to resistance and vaccine escape. Both species share the ability to become naturally competent, and in this context, competence-associated killing mechanisms such as fratricide are thought to play an important role in interspecies gene exchange. Here, we explore the general mechanism of natural genetic transformation in the two species and touch upon the fundamental clinical and evolutionary implications of sharing similar competence, fratricide mechanisms, and a large fraction of their genomic DNA.

Highlights

Natural genetic transformation is a prime driver of evolution by promoting genetic variability through horizontal gene transfer (HGT) in both Gram-positive and Gram-negative bacteria
The ability to exchange genetic material via natural transformation is specially conserved across the genus Streptococcus, and is recognized to play an important role in the evolution of their genomes (Straume et al, 2015)
There is evidence of extensive recombination events within the cluster S. pneumoniae/S. pseudopneumoniae/S. mitis during their parallel evolution, which reflects their similarities. This exchange has been demonstrated mostly as a “one-way street,” with S. mitis and S. oralis as the DNA providers and S. pneumoniae as the recipient, even though a large proportion of S. mitis and S. oralis strains have the complete set of genes required for transformation (Havarstein et al, 1997; Kilian et al, 2008)

Summary

INTRODUCTION

Natural genetic transformation is a prime driver of evolution by promoting genetic variability through horizontal gene transfer (HGT) in both Gram-positive and Gram-negative bacteria. Streptococci represent an extremely diverse group of more than 100 species, which colonize both humans and animals. Their interactions with the host, range from commensalistic to pathogenic relationships. There is evidence of extensive recombination events within the cluster S. pneumoniae/S. pseudopneumoniae/S. mitis during their parallel evolution, which reflects their similarities. This exchange has been demonstrated mostly as a “one-way street,” with S. mitis and S. oralis as the DNA providers and S. pneumoniae as the recipient, even though a large proportion of S. mitis and S. oralis strains have the complete set of genes required for transformation (Havarstein et al, 1997; Kilian et al, 2008). We briefly review the known mechanism of natural transformation in the mitis group and explore the evolutionary and clinical implications of the close relatedness and common features shared by S. pneumoniae and its nearby relatives

CLOSE RELATIVES

THE COMPETENT STATE

OF CHAOS

FRATRICIDE AND ITS IMPLICATIONS

IN THE MITIS GROUP

FUTURE PERSPECTIVES