Gram-positive bacteria are encased in a thick cell wall consisting of a complex and dynamic assembly of several glycopolymers (peptidoglycan, polysaccharides, teichoic acids) and proteins. It is a rigid structure to maintain cell shape and integrity but also a flexible one to accommodate cell growth and division and to face changing environmental conditions. Furthermore, components exposed at the cell surface mediate the interactions of bacteria with their surroundings in complex environments such as food fermentation systems or the gastrointestinal tract. Our research follows three main objectives:
To achieve these goals, we combine biochemical and genetic approaches. In particular, we have developed original and robust procedures coupled with powerful analytical techniques (chromatography, mass spectrometry) and an original cell wall mutant selection method.
Our bacterial models are beneficial bacteria, lactic acid bacteria used in food fermentations (lactococci, lactobacilli), or commensal bacteria of the intestinal microbiota with positive health effects on their host.
The roles of the so-called secondary cell wall glycopolymers including polysaccharides that functionalize the peptidoglycan layer in Gram-positive bacteria, are multiple but not fully elucidated. We have unraveled the structure and role of cell wall polysaccharides (CWPS) in lactococci that are cocci with an ovoid shape and model lactic acid bacteria. Lactococci synthesize complex rhamnose-rich cell wall polysaccharides (Rha-CWPS) covalently bound to peptidoglycan. These CWPS are made of a conserved component (a rhamnan chain) and a variable one exposed at the bacterial surface (named the pellicle, PSP). We have deciphered the original biosynthetic pathway underlying the structural diversity of these Rha-CWPS.
We have shown that rhamnan is an essential component of the cell wall whereas the pellicle is a key component for the proper functioning of the cell division machinery. From a more applied perspective, Rha-CWPS are of particular interest in lactococci since they are the receptors of the major families of bacteriophages infecting lactococci, which threaten milk fermentations. In collaboration with University College Cork and CNRS-Marseille, we have shown that the structural diversity of the surface pellicle between strains explains the narrow host range specificity of these bacteriophages.
Cell wall polysaccharides also appear to be used as receptors by bacteriophages infecting several other species of lactic acid bacteria including the wine lactic acid bacterium Oenococcus oeni. We are currently involved in their biochemical and functional characterization in a collaborative ANR project with CNRS-Marseille, ISVV-Bordeaux, ULCO-Boulogne-sur-mer, and the PAGés platform (Lille).
Exopolysaccharides (EPS) synthesized by certain lactic acid bacteria are secreted into the surrounding medium. They are traditionally known in the dairy industry for their contribution to the texture of fermented milk products. The diversity of bacterial EPS structures is enormous. Since their physicochemical properties depend on their structure, the spectrum of technological and functional properties of EPS is extensive in the food, cosmetic, or biomedical fields.
We have developed an innovative strategy for isolating non-GMO EPS-overproducing mutants, based on differential sedimentation during bacterial growth in semi-liquid medium conditions. Spontaneous mutants producing markedly high amounts of EPS have been obtained from various food and probiotic lactobacilli strains and species. The potential of these EPS as pre- or probiotics or for cosmetic applications are under investigation.
In addition to their technological interest, selected EPS-overproducing strains are valuable tools for studying the biosynthesis and anchoring/secretion of polysaccharides in lactobacilli as well as the mechanisms leading to EPS overproduction.
Bacterial cell wall components or derived fragments are considered crucial molecular effectors of the cross-talk between beneficial bacteria present in the gut microbiota and their host, maintaining intestinal homeostasis. We are analyzing the structure and modifications of peptidoglycan in bacterial species of the intestinal microbiota in relation to their beneficial effects on their host through activation of the NOD2 receptor.
Teichoic acids and their structural variations are also players in the dialogue between bacteria and host. In close collaboration with ENS-Lyon and MMBS-Lyon in the frame of an ANR project, we have characterized DltE, a novel actor of the D-alanylation machinery involved in modulating D-alanine substituents on teichoic acids in the cell wall of Lactiplantibacillus plantarum, a dominant species of the Drosophila gut microbiota. Moreover, D-alanylated lipoteichoic acids were shown to be direct cues that promote juvenile growth in Drosophila during chronic undernutrition.
The complete list of our publications here.
