Abstract: The cell wall is a three-dimensional sugar and peptide network that surrounds the bacterial cell. It confers a cell shape adapted to the ecological niche of the bacterium and protects it against mechanical stress exerted by the environment. Cell wall synthesis and integrity are thus essential for bacterial proliferation and survival. Despite the importance of these fundamental processes, which constitute sources of antibiotic targets, we still poorly understand how the cell wall is assembled and remodelled in space and time to ensure proper cell division, shape and integrity. This is particularly true for ovoid-shaped bacteria such as streptococci and enterococci, in which two different modes of cell wall synthesis, dedicated to cell division and elongation, are confined to an annular region with nanometric dimensions at midcell.

Fluorescence microscopy is a method of choice to investigate cell wall assembly but it suffers from two major drawbacks. First, the newly synthesized material must be labelled with a probe that will not perturb the physiological process. Second, the physical properties of light limit the resolution to about 250 nm, which approximates the dimensions of the cell wall synthesis region. We have met these two challenges by combining metabolic cell wall labelling (using click chemistry) and super-resolution fluorescence microscopy (dSTORM) in the ovoid-shaped human pathogen Streptococcus pneumoniae. Our nanoscale-resolution data ( 30 nm) unravelled unprecedented spatio-temporal features of cell wall assembly and fate along the cell cycle. It provided geometrical and kinetic parameters of cell wall synthesis that we further used to simulate the morphogenesis of the ovoid cell in silico.

I will present our methodological strategy and the major insights that our experimental and modelling analyses revealed into cell wall synthesis and morphogenesis in ovococci.

Biography: I started studying morphogenesis and division processes in Streptococcus pneumoniae during my Ph.D. at the Institute for Structural Biology (2000-2003, IBS, Grenoble, FR). Under the supervision of Thierry Vernet, I investigated the cellular localization of the Penicillin-Binding Proteins and the structure of the cell wall hydrolase DacA.

For my 1st post-doc, I joined the Grenoble EMBL outstation in Stephen Cusack’s group (2004-2007, EMBL, Grenoble, FR) where I characterized the structure of a protein complex involved in neuron development in humans.

For my 2nd post-doc, I joined the group of David Rudner at Harvard Medical School (2007-2010, HMS, Boston, USA) to study the function of two macromolecular complexes (SpoIIIA-SpoIIQ and SpoIIP-SpoIID) during sporulation in Bacillus subtilis.

I was recruited in 2010 as a CNRS researcher and since January 2021, I took the head of the Pneumococcus group at the IBS (Grenoble, France). My group employs complementary techniques in microbial genetics, biochemistry, structural biology (crystallography, electron microscopy) and cell imaging (including super-resolution fluorescence microscopy) to study the mechanisms of cell morphogenesis during vegetative growth and sporulation. We address in particular the synthesis of the two main components of the cell wall in the human pathogen S. pneumoniae, the peptidoglycan and the teichoic acids. In parallel, we study the structure and the function of the SpoIIIA-SpoIIQ complex, a putative new type of secretion system required for spore development in B. subtilis.