The RecA-directed recombination pathway of natural transformation initiates at chromosomal replication forks in the pneumococcus.

Homologous recombination (HR) is a crucial mechanism of DNA strand exchange that promotes genetic repair and diversity in all kingdoms of life. Bacterial HR is driven by the universal recombinase RecA, assisted in the early steps by dedicated mediators that promote its polymerization on single-stranded DNA (ssDNA). In bacteria, natural transformation is a prominent HR-driven mechanism of horizontal gene transfer specifically dependent on the conserved DprA recombination mediator. Transformation involves internalization of exogenous DNA as ssDNA, followed by its integration into the chromosome by RecA-directed HR. How DprA-mediated RecA filamentation on transforming ssDNA is spatiotemporally coordinated with other cellular processes remains unknown. Here, we tracked the localization of fluorescent fusions to DprA and RecA in Streptococcus pneumoniae and revealed that both accumulate in an interdependent manner with internalized ssDNA at replication forks. In addition, dynamic RecA filaments were observed emanating from replication forks, even with heterologous transforming DNA, which probably represent chromosomal homology search. In conclusion, this unveiled interaction between HR transformation and replication machineries highlights an unprecedented role for replisomes as landing pads for chromosomal access of tDNA, which would define a pivotal early HR step for its chromosomal integration.

Pneumococcal competence is a populational health sensor driving multilevel heterogeneity in response to antibiotics.

Competence for natural transformation is a central driver of genetic diversity in bacteria. In the human pathogen Streptococcus pneumoniae, competence exhibits a populational character mediated by the stress-induced ComABCDE quorum-sensing (QS) system. Here, we explore how this cell-to-cell communication mechanism proceeds and the functional properties acquired by competent cells grown under lethal stress. We show that populational competence development depends on self-induced cells stochastically emerging in response to stresses, including antibiotics. Competence then propagates through the population from a low threshold density of self-induced cells, defining a biphasic Self-Induction and Propagation (SI&P) QS mechanism. We also reveal that a competent population displays either increased sensitivity or improved tolerance to lethal doses of antibiotics, dependent in the latter case on the competence-induced ComM division inhibitor. Remarkably, these surviving competent cells also display an altered transformation potential. Thus, the unveiled SI&P QS mechanism shapes pneumococcal competence as a health sensor of the clonal population, promoting a bet-hedging strategy that both responds to and drives cells towards heterogeneity.

Presence of nonhemolytic pneumolysin in serotypes of Streptococcus pneumoniae associated with disease outbreaks.

Pneumolysin is an important virulence factor of the human pathogen Streptococcus pneumoniae. Sequence analysis of the ply gene from 121 clinical isolates of S. pneumoniae uncovered a number of alleles. Twenty-two strains were chosen for further analysis, and 14 protein alleles were discovered. Five of these had been reported previously, and the remaining 9 were novel. Cell lysates were used to determine the specific hemolytic activities of the pneumolysin proteins. Six strains showed no hemolytic activity, and the remaining 16 were hemolytic, to varying degrees. We report that the nonhemolytic allele reported previously in serotype 1, sequence type (ST) 306 isolates is also present in a number of pneumococcal isolates of serotype 8 that belong to the ST53 lineage. Serotype 1 and 8 pneumococci are known to be associated with outbreaks of invasive disease. The nonhemolytic pneumolysin allele is therefore associated with the dominant clones of outbreak-associated serotypes of S. pneumoniae.