Maxime Ardré

See our paper on pyoverdin localization at the cell’s pole.

Title: Polar accumulation of pyoverdin and exit from stationary phase

Authors: Clara Moreno Fenoll, Maxime Ardré, Paul B Rainey

DOI: 10.1093/femsml/uqae001

Bacteria secrete extracellular products that enable nutrients to be obtained from the environment. A secreted product of relevance for medicine, agriculture, and biotechnology is the iron-chelating siderophore, pyoverdin, which is produced by members of the genus Pseudomonas. By analyzing the behavior of single cells we show that on cessation of cell division, pyoverdin localizes to cell poles, but is then released to the environment prior to resumption of cell growth. Of particular significance is the ecological relevance of this behavior: cells that accumulate the siderophore resume growth with minimal delay. Our study reveals a hitherto unrecognized dimension to the biology of Pseudomonas that may prove central to understanding the broader ecological and physiological relevance of pyoverdin.

A first approach is to monitor the bacteria at the single cell level. To do so I developed a microfluidic chip integrated to a microscope in order to control the chemical environment of the bacteria. The microfluidic technology sketched in the image above allows to keep a steady environment at the bacterial scale. Thanks to this device we can follow the growth of microcolonies such as in the movie presented here.

From this movie big data analysis (neuronal network) can be performed to segment the images and extract valuable informations from the dynamical system.

The top part of the movie shows the bacteria producing an important molecules. The bottom part of the movie shows the shape of the same bacteria. We can see that the rod shape bacteria grow and split themselves with time to form microcolonies (few micrometers).  In the same time the production of the important molecule is specific to each bacteria and “shared” between neighboors.

The relationship between the number of cells colonizing a new environment and time for resumption of growth is a subject of long-standing interest. In microbiology this is known as the “inoculum effect.” Its mechanistic basis is unclear with possible explanations ranging from the independent actions of individual cells, to collective actions of populations of cells. Here, we use a millifluidic droplet device in which the growth dynamics of hundreds of populations founded by controlled numbers of Pseudomonas fluorescens cells, ranging from a single cell, to one thousand cells, were followed in real time. Our data show that lag phase decreases with inoculum size. The decrease of average lag time and its variance across droplets, as well as lag time distribution shapes, follow predictions of extreme value theory, where the inoculum lag time is determined by the minimum value sampled from the single-cell distribution. Our  experimental results show that exit from lag phase depends on strong interactions among cells, consistent with a “leader cell” triggering end of lag phase for the entire population.

Title: A leader cell triggers end of lag phase in populations of Pseudomonas fluorescens

Authors : Maxime Ardré, Guilhem Doulcier, Naama Brenner, Paul B Rainey

DOI: 10.1093/femsml/uqad040

See our paper on biofilms and the pathway controlling the cellulose production at the air-liquid-interface.

Title: Causes and biophysical consequences of cellulose production by Pseudomonas fluorescens SBW25 at the air-liquid interface
Authors: Maxime Ardré, Djinthana Dufour, Paul B Rainey

Biofilm formation by P. fluorescens at the air-liquid interface (ALI). The photo is a top view of a well (40mm diameter).
The bacteria produce small white dots on the interface by up regulating the cellulose production locally. The more white areas scattered on the surface indicate the presence of active convection into the liquid below. This convection is driven by the biomass increase at the ALI.
We decipher the importance of several metabolic pathways involved in the cellulose production. We give a mathematical model that explains the apparition of streams. We rationalize the consequences of the streams over the spatial distribution of extracellular products produced by the bacteria within the biofilm.

“The bacterium Bacillus subtilis frequently forms biofilms at the interface between the culture medium and the air. We present a mathematical model that couples a description of bacteria as individual discrete objects to the standard advection-diffusion equations for the environment. The model takes into account two different bacterial phenotypes. In the motile state, bacteria swim and perform a run- and-tumble motion that is biased toward regions of high oxygen concentration (aerotaxis). In the matrix-producer state they excrete extracellular polymers, which allows them to connect to other bacteria and to form a biofilm. Bacteria are also advected by thefluid, and can trigger bioconvection. Numerical simulations of the model reproduce all the stages of biofilmformation observed in laboratory experiments. Finally, we study the influence of various model parameters on the dynamics and morphology of biofilms.”

Title: An individual-based model for biofilm formation at liquid surfaces

Authors: Maxime Ardré, Hervé Henry , Carine Douarche and Mathis Plapp