Christophe Place

Counterclockwise Circular Motion of Bacteria Swimming at the Air-Liquid Interface

Flagellar propulsion of swimming Escherichia coli produces circling clockwise motions near planar solid surfaces. Counterclockwise motion was first reported near air-TN medium interfaces, showing that slip at the interface is a key parameter of bacterial swimming.

A Model of Sequence-Dependent Protein Diffusion Along DNA

We introduce a probabilistic model for protein sliding motion along DNA during the search of a target sequence. The model accounts for possible effects due to sequence-dependent interaction between the nonspecific DNA and the protein. Hydrogen bonds formed at the target site are used as the main sequence-dependent interaction between protein and DNA. The resulting dynamical properties and the possibility of an experimental verification are discussed in details. We show that, while at large times the process reaches a linear diffusion regime, it initially displays a sub-diffusive behavior. The sub-diffusive regime can last sufficiently long to be of biological interest.

Curvature reversal of the circular motion of swimming bacteria probes for slip at solid/liquid interfaces

The orientation of the circular motion of most swimming E. coli at a solid surface is shown to be (i) reversed by supplementing the medium with alginate and (ii) restored by rendering the glass surface positively charged. Given slip promotion by the polymer and suppression by its anchoring on the surface, observations are interpreted by slip at the boundary. This points out bacterial swimming as a probe of the solid/fluid boundary condition.

Synthesis of multifunctional lipid–polymer conjugates: application to the elaboration of bright far-red fluorescent lipid probes

A new class of lipid-ended polymer conjugates presenting reactive sites regularly distributed along the polymer chain were synthesized using RAFT polymerization. The chosen modular approach enables preparation of different lipid families by tuning the nature of the phospholipid α-end, the molecular weight and the lateral functions of the polymer chain. The multiple activated ester functions of the conjugates can indeed be used for the efficient coupling of a great variety of amino-containing entities of interest. In this study, we elaborated original fluorescent lipid–polymer probes for optical microscopy by coupling along the chain a controlled number of chromophores emitting in the far-red where auto-fluorescence and light absorption by biological samples are limited. Water-soluble fluorescent lipid probes exhibiting an enhanced brightness were obtained. As a proof of concept, these probes were able to efficiently label the lipid bilayer of liposomes of various sizes. Such multifunctional lipid-ended polymers thus exhibit great potential to functionalize model and natural lipid assemblies.

Expression of Nucleolin Affects Microtubule Dynamics

Nucleolin is present in diverse cellular compartments and is involved in a variety of cellular processes from nucleolar structure and function to intracellular trafficking, cell adhesion and migration. Recently, nucleolin has been localized at the mature centriole where it is involved in microtubule nucleation and anchoring. Although this new function of nucleolin linked to microtubule regulation has been identified, the global effects of nucleolin on microtubule dynamics have not been addressed yet. In the present study, we analyzed the roles of nucleolin protein levels on global microtubule dynamics by tracking the EB3 microtubule plus end binding protein in live cells. We have found that during microtubule growth phases, nucleolin affects both the speed and life time of polymerization and by analyzing catastrophe events, we showed that nucleolin reduces catastrophe frequency. This new property of nucleolin was then confirmed in a cold induced microtubule depolymerization experiment in which we have found that cold resistant microtubules were totally destabilized in nucleolin depleted cells. Altogether, our data demonstrate a new function of nucleolin on microtubule stabilization, thus bringing novel insights into understanding the multifunctional properties of nucleolin in healthy and cancer cells.

Far-Red Fluorescent Lipid-Polymer Probes for an Efficient Labeling of Enveloped Viruses.

Far‐red emitting fluorescent lipid probes are desirable to label enveloped viruses, for their efficient tracking by optical microscopy inside autofluorescent cells. Most used probes are rapidly released from membranes, leading to fluorescence signal decay and loss of contrast. Here, water‐soluble lipid‐polymer probes are synthesized harboring hydrophilic or hydrophobic far‐red emitting dyes, and exhibiting enhanced brightness. They efficiently label Hepatitis C Virus pseudotyped particles (HCVpp), more stably and reproducibly than commercial probes, and a strong fluorescence signal is observed with a high contrast. Labeling with such probes do not alter virion morphology, integrity, nor infectivity. Finally, it is shown by fluorescence microscopy that these probes enable efficient tracking of labeled HCVpp inside hepatocarcinoma cells used as model hepatocytes, in spite of their autofluorescence up to 700 nm. These novel fluorescent lipid‐polymer probes should therefore enable a better characterization of early stages of infection of autofluorescent cells by enveloped viruses.

Host cell surfaces induce a Type IV pili-dependent alteration of bacterial swimming

For most pathogenic bacteria, flagellar motility is recognized as a virulence factor. Here, we analysed the swimming behaviour of bacteria close to eukaryotic cellular surfaces, using the major opportunistic pathogen Pseudomonas aeruginosa as a model. We delineated three classes of swimming trajectories on both cellular surfaces and glass that could be differentiated by their speeds and local curvatures, resulting from different levels of hydrodynamic interactions with the surface. Segmentation of the trajectories into linear and curved sections or pause allowed us to precisely describe the corresponding swimming patterns near the two surfaces. We concluded that (i) the trajectory classes were of same nature on cells and glass, however the trajectory distribution was strikingly different between surface types, (ii) on cell monolayers, a larger fraction of bacteria adopted a swimming mode with stronger bacteria-surface interaction mostly dependent upon Type IV pili. Thus, bacteria swim near boundaries with diverse patterns and importantly, Type IV pili differentially influence swimming near cellular and abiotic surfaces.