PhD Defense:

The effect of membrane fluidity on cell wall shape and elongation in Bacillus subtilis

The spatial organization of molecules is essential for cellular life. Lipids play a key role in forming membranes that enclose cell contents and act as selective barriers between the external environment and the components of cellular metabolism. Due to their broad physico-chemical diversity, lipids can create regions with different fluidity within the membrane, helping to organize specific proteins and ensure their proper function. This process has only recently been described in bacteria, and while bacterial membrane domains are not yet well understood, they seem to be involved in important processes like cell morphogenesis. In bacteria, cell shape is maintained by the cell wall, an external layer mainly composed of peptidoglycan. The synthesis of peptidoglycan during cell wall elongation is controlled by two semi-autonomous systems: the Rod complex and class A Penicillin-Binding Protein (aPBP) polymerases. Recent research suggests that proteins in the Rod complex, particularly bacterial actin MreB, are associated with regions of increased fluidity (RIFs) in the membrane. This thesis explores how membrane fluidity affects cell shape and elongation in the Gram-positive model bacterium Bacillus subtilis, offering new insights into the relationship between membrane properties and bacterial cell structure.

Keywords: Bacillus subtilis; cell wall elongation; plasma membrane fluidity; exogenous fatty acids; Total Internal Reflexion Fluorescence (TIRF) microscopy; Fluorescence Correlation Spectroscopy (FCS); MreB; PBP1.

Jury members:

• Christophe GRANGEASSE (CNRS, Université de Lyon) – Rapporteur
• Arnauld SERGÉ (INSERM, Université Aix-Marseille) – Rapporteur
• Nienke BUDDELMEIJER (Institut Pasteur, Université Paris Cité) – Examinatrice
• Frédérique POMPEO (CNRS, Université Aix-Marseille) – Examinatrice

Directed by:

Rut CARBALLIDO-LÓPEZ (Micalis Institute, INRAe Jouy-en-Josas/Anthony, AgroParisTech, Université Paris-Saclay) and Charles KERVRANN (INRIA Rennes, Curie Institute)

Supervised by:

Cyrille BILLAUDEAU (Micalis Institute, INRAe Jouy-en-Josas/Anthony, AgroParisTech, Université Paris-Saclay).