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Dissecting a new mechanisms of bacterial cell-cell communication

Final Report Summary - MICROBIAL SENSING (Dissecting a new mechanisms of bacterial cell-cell communication)

In order to adapt to environmental changes and form multicellular structures like biofilms and fruiting bodies, bacteria must be able to perceive the environment and communicate with each other. For the project “Microbial Sensing”, we proposed to investigate the involvement of sensory apparatuses termed Chemosensory Systems (CSS) in the mechanisms of bacterial cell-cell communication and fruiting body formation using the gliding bacterium Myxococcus xanthus as a model system.
During the past four years, we obtained findings that we reported in three manuscripts, one published and two ready for submission. First, we characterized all M. xanthus CSS by a multidisciplinary approach. We found that M. xanthus contains eight predicted CSS and 21 chemoreceptors. We systematically deleted genes encoding components of each CSS and chemoreceptors and determined their effects on M. xanthus social behaviors. Then, we used a multidisciplinary approach including phylogenetics, fluorescence microscopy and protein-protein interaction assays to identified a large chemosensory module formed by three interconnected CSS and multiple chemoreceptors. Finally, we showed that complex behaviors such as cell group motility and biofilm formation require regulatory apparatus composed by multiple interconnected Che-like systems (Moine et al., PLoS Genetics, 2014).
Second, in a deeper analysis of M. xanthus Che proteins, we remarked the presence of a small CheW-like protein (FrzB) divergent from the canonic counterparts. Protein structure modeling in collaboration with Xavier Morelli and Philippe Roche (iSCB, CRCM, CNRS Marseille), protein interaction assays and phenotypic analyses revealed that FrzB does not function like a real CheW in the signal transduction cascade, but it delivers signals to the Frz cytoplasmic chemosensory system (Guiseppi et al., manuscript ready for submission).
Third, we showed that the FrzCD cytoplasmic chemoreceptor forms chemosensory protein clusters by binding to the bacterial nucleoid via a N-terminal protein-domain similar to that observed in eukaryotic histones. During cell division, clusters are not evenly segregated between the two daughter cells, which results in variations in the number of FrzCD clusters from cell to cell within the bacterial population. These variations in the number of clusters generates a phenotypic noise in turn important for M. xanthus social behaviors. The phenotypic noise is important in bacteria to deploy specialized cells in anticipation of possible adverse changes in the environment. The phenotypic noise can arise from multiple sources, such as variations in the activity of individual genes or from cell-to-cell variations in metabolic activity, or from fluctuating levels of an external signal. With this project, we show for the first time that phenotypic noise can also be reached through stochastic and uneven segregation events. By proposing a function of the bacterial nucleoid in the organization of cytoplasmic chemosensory systems and generation of phenotypic noise, this work opens new perspectives on the role of nucleoid in the general functional architecture of the bacterial cell (Moine et al., manuscript ready for submission).

In addition to the results mentioned above, we also developed three innovative techniques: a microfluidic device to generate stable gradients coupled to a microscope to study the response of individual cells to chemicals (in collaboration with Dr. Olivier Theodolys, CNRS UMR 6212, Adhésion et Inflammation); a high-throughput system to study the protein subcellular localization in vivo by the aid of a robot (Tecan) coupled to a fluorescence microscope (in collaboration with Dr. Leon Espinosa, CNRS UMR 7283, Laboratoire de Chimie Bactèrienne); a high-throughput screening technique to analyze the phenotypic effect of hundreds of molecules on M. xanthus social behaviors (in collaboration with Dr. Leon Espinosa, CNRS UMR 7283, Laboratoire de Chimie Bactèrienne).

The study of bacterial communities is of biomedical and ecological importance as the formation of highly organized multicellular structures such as biofilms and fruiting bodies render infectious bacteria resistant to antibiotics and are very difficult to treat. Essential for biofilm and fruiting body formation is the ability of bacterial cells to sense the environment and communicate with each other. Uncovering how M. xanthus cell communicate and perceive the environment through its arsenal of CSS will help identifying new targets of antibiotics in the treatment of biofilm mediated infections. Moreover, we believe that this project will bring an important contribution to the field that studies bacterial communities as it will provide the link between the bacterial cell architecture, the functional organization of complex signal transduction arrays and cell behaviors.