Monday, 13th December 2021
Elise Courtais will present the results of her PhD work on study of the cell division arrest induced during competence for natural genetic transformation in Streptococcus pneumoniae
Natural genetic transformation is a programmed mechanism of horizontal gene transfer in bacteria. It requires the entry of cells into a transient physiological state called competence, during which a large membrane-associated multiprotein complex is synthesized and ensures binding, internalization and integration by homologous recombination of exogenous DNA into the recipient genome. In the human pathogen Streptococcus pneumoniae, this machinery assembles at the cytoplasmic membrane and across the cell wall at midcell, the division site. In this species, competence develops in every cells of an exponentially growing culture and causes a growth arrest phenotype, suggesting a functional link between competence development and the cell cycle. During my thesis, I investigated the mechanism of this growth arrest.
I first tested the involvement of different components of the transformation machinery, localized at the division site, in the growth arrest observed during the induction of competence. This research allowed me to demonstrate that the assembly of the transformation pilus, responsible for the capture of exogenous DNA, leads to the aggregation of competent cells. My results suggest that this aggregation plays a role in fratricide, a process specific to S. pneumoniae in which competent cells develop the ability to “kill” non-competent sister cells.
I also adapted a super-resolution microscopy technique, with the aim of understanding the molecular mechanism of the inhibition of cell division in competent cells. I have developed tools allowing 3D- PALM (PhotoActivated Localization Microscopy) acquisitions, a microscopy technique based on single molecule localization with a resolution of about 50 nm, to determine the structure of the FtsZ ring in 3 dimensions. To study the fate of the FtsZ ring during the different stages of the cell cycle, I measured the morphological parameters of each cell from phase contrast images. My first results show that the FtsZ proteins form a heterogeneous ring with dimensions ranging from 300 to 700 nm in diameter, while the thickness (~110 nm) and depth (~100 nm) remain constant during the cell cycle. The presence of areas of high fluorescence intensity and areas of lower intensity indicate that the proteins are not homogeneously distributed in the ring but are grouped as clusters of varying density.