Biophysics of growth

The initial step of several human or plant fungal infections requires active penetration of host tissue. For example, active penetration of intestinal epithelia by Candida albicans is critical for dissemination from the gut into the bloodstream. In this project (in collaboration with R. Arkowitz Team in iBV, Institut de Biologie Valrose), we have used PDMS microfabricated array of wells to probe the ability of filamentous C. albicans cells to penetrate and grow invasively in substrates of different stiffness. We show that there is a stiffness threshold for penetration and that invasive growth within a stiff substrate is characterized by dramatic filament buckling, along with a stiffness-dependent decrease in extension rate. Our results show that cell growth and morphology are altered during invasive growth, suggesting stiffness dictates the host cells that C. albicans can penetrate.
In addition to the fabrication and experiments on living matter, we have developed an experiment to probe the mechanical effect of penetration of a rigid object in PDMS. We have looked at it, using two different setups at two different scales (millimetric and tens of micrometers in diameter for the indenter).
 

• Puerner et al., BMC Biology 18(1) 122 (2020), 10.1186/s12915-020-00833-0
• N. Kukhaleishvili, PhD thesis (2020) : Biophysique de la croissance filamenteuse fongique et mécanique de perforation dans des élastomères.
• News from INSB-CNRS: https://www.insb.cnrs.fr/fr/cnrsinfo/confinement-force-biophysique-de-linvasion-fongique

Bacillus subtilis is a model bacterium to study different biological and physical properties of biofilms. Living mainly in the soil, it preferentially forms biofilms at solid/air or liquid/air interfaces. B. subtilis biofilms at a solid/air interface form macro-colonies and are traditionally studied on agar plates. We develop an experiment to explore biofilm collective motility in contact with external gradients of osmotic pressure. To produce stable osmotic gradients in agar gels, we use a custom-made setup through millifluidics. Biofilms respond to the external gradient by developing an asymmetric shape, consistent with the expectations.

• M. Iapichino, PhD thesis (2019): Individual and collective motility in microbial systems: bacterial biofilms and fungal spore dispersal

Using microfluidic techniques, we are able to trap one or a few bacteria in microscopic water drops (dispersed in oil) to study properties at the individual or small community level for a large number of samples in parallel.
It is possible to characterize the growth of microorganisms in invert emulsion droplets, by measuring the volume of the droplets over time, this volume depending on osmotic phenomena.
The objective of this experiment is to design a set-up to generate aqueous droplets in a compact stack in an oil phase, to encapsulate bacteria in it in order to study the relationship between population growth and droplet size. The consumption of nutrients during bacterial growth and the production of extracellular matrix polymers have opposite effects on osmotic pressure, and these effects can then be decoupled.