Bacterial respiration is the central theme of our research activity. What are the molecular determinants of the reactivity of respiratory complexes towards their substrates, how to ensure optimal connectivity of electron transfer chains through a spatial and dynamic organization of bioenergetic complexes or how variations in the expression of respiratory actors can give rise to phenotypic heterogeneity at the population level. These are all fascinating questions that we address by taking advantage of the study of several bacterial models, namely Escherichia coli, Bacillus subtilis and lactic acid bacteria, and the use of different scales of observation, from the molecular to the cellular, and the population level.
By combining multidisciplinary approaches at the interface of biology, chemistry and physics down to the cellular level, we aim to provide new insights into the understanding of bacterial physiology.
In this study, published in the journal ACS Catalysis, the group of Axel Magalon together with Guigliarelli’s team (BIP) and JP. Duneau (LISM) show that a
Cofactor-binding metalloproteins of the Mo/W-bisPGD type are widely distributed in prokaryotes and display an extraordinary diversity and richness both in terms of their modular structural organization and their reactivity towards substrates ranging from the simple inorganic molecule CO2 to molecules considered as xenobiotics. Life on Earth has taken advantage of this plasticity by involving molybdoenzymes in multiple metabolic pathways and particularly in respiration in microorganisms.
Our focus is on several molybdenum enzyme complexes (e.g. nitrate reductase, formate dehydrogenase). Interestingly, a conundrum exists in this enzyme superfamily when considering the limited chemical diversity of Mo/W active sites and a yet uncovered diversity of reactivities and substrates. The discovery of a new class of Mo-enzymes catalyzing the conversion of formate to CO2 leads us to identify the molecular determinants controlling the reactivity and directionality of these biocatalysts with high biotechnological potential (Fig. 1). To this end, a multidisciplinary approach including bioinformatics, biochemistry, enzymology, mutagenesis and spectroscopies is employed thanks to several collaborations.
Fig1 : B. subtilis genome encodes putative non-canonical FDHs (from Arias-Cartin et al 2021 JBC)
These bioenergetic complexes have in common that they react with quinones, electron and proton shuttles diffusing in the lipid bilayer and ensuring the connectivity of electron transfer chains. Bioenergetic enzymes differ with respect to how they react to the presence of variable types of quinones at their well-defined binding sites. We aim to understand how these enzymes adapted to various quinones using similar above-mentioned approaches (Fig. 2).
Fig2 : Deciphering the interaction mode of quinones in a bacterial respiratory complex (from Seif Eddine et al 2020 BBA and Seif Eddine et al 2017 ChemPhysChem)
To date, the high level of knowledge about the structure and function of bioenergetic complexes contrasts sharply with the poor understanding of the relationship between the distribution and diffusion of respiratory components and the efficiency of the process. Recently, we evidenced that two bioenergetic complexes are organized into nanodomains enriched at the cell poles in E. coli when both are involved in an electron transport chain dedicated to nitrate utilization and contributing to cell growth. Our working hypothesis is that clustering of bioenergetic complexes into nanodomains facilitates the turnover of quinones and explains the efficient functioning of bacterial respiration. To this end, we combine spatially and temporally resolutive high end fluorescence microscopy approaches at the single cell level with physiological, kinetics and physico-mathematical approaches (Fig. 3).
Fig3 : Spatial distribution of respiratory complexes in Escherichia coli (from Alberge et al 2015 eLife and Bulot et al 2019 mBio)
To cope with sudden environmental changes, bacteria can use strategies relying on the generation of subpopulations exhibiting different metabolic features. One of these evolutionary strategies, called bet-hedging, enables a population to prepare for a potential switch to a new environmental condition by harboring a subpopulation that is pre-adapted to the new environment (Fig. 1).
Fig1 : The « bet-hedging » organization. Only the cells expressing the required factor in a sufficient amount can expand immediately when the environmental conditions change.
Lactic acid bacteria provide a useful model to understand the functioning of bioenergetic electron transfer chains as they present a minimal set of bioenergetic complexes involved in oxygen and / or nitrate respiration. Their auxotrophy regarding heme, the essential cofactor of terminal oxidases and reductases, and in some cases quinones, questions the physiological role of respiration in these traditionally fermenting bacteria, and the rationale for constitutive expression of respiratory complexes (Fig. 2). Our working hypothesis is that phenotypic heterogeneity is an explanation of how LAB populations are able to rapidly take advantage of the presence of heme (and quinones) without costing too much to the whole population. Using single cell approaches, including fluorescence-guided cytometry and microfluidic time-lapse microscopy, we are studying the phenotypic heterogeneity within Lactococcus lactis and Lactiplantibacillus plantarum populations regarding the expression and activity of the actors of their electron transfer chains.
Fig2 : Energetic metabolism in Lactic Acid Bacteria. Lactic acid bacteria are fermenting bacteria that produce lactic acid as their major metabolic end product. They are also able to respire when heme and for some species menaquinones are supplied by the environment.
Group leader / Research director (DR-CNRS)
Assistant professor (MCF-AMU)
Assistant engineer (AI-CNRS)
Assistant professor (MCF-AMU)
PhD student (PhD-AMU)