PhD Discussions Session: Maria Chiara Biagi and Roger Oria

Nanoscale dielectric characterization of single bacterial cells at microwave frequency

Information on the microwave electromagnetic properties of cell suspensions and tissues has already led to important application in therapeutic and diagnostic. In recent years, a new microscopy technique has appeared, able to resolve the electromagnetic response at GHz even further down, at nanoscale spatial resolution: Scanning Microwave Microscope (SMM). Its application to single cells would possibly allow not just to scale down the existing medical and biological techniques, but would also give rise to a new class of label-free imaging methods based on dielectric contrast. Yet, the quantification of the intrinsic dielectric properties (i.e. complex permittivity) of non-planar irregular shaped objects like single cells from the standard SMM images remains a challenge, because the experimental signal is greatly affected by the huge presence of non-local contributions.

We developed a methodology to quantify and remove them, which consequently enables to obtain images related only to the intrinsic dielectric response of the sample. These images are then suitable for a quantitative analysis and, in combination with 3D finite element numerical calculations, a map of the complex permittivity of the cell can be obtained.
We have applied this procedure to a single bacterial cell (E. coli) and quantified for the first time its complex permittivity at ~19 GHz, in dry and humid conditions.

Interplay between integrin expression, clustering, and substrate rigidity in cell mechanical response

Essential cell functions such as proliferation, differentiation, or migration are determined by the rigidity and composition of the extracellular matrix (ECM). Understanding this interaction requires a precise control of ECM mechanical properties and molecular distribution of cell-ECM ligands, as well as the ability to measure the mechanical forces transmitted at the cell-ECM interface. To address this issue, we have developed an approach based on polyacrylamide substrates of tunable rigidity decorated with nanometric regular hexagonal patterns of RGD ligands, which serve as binding sites for single integrins. By using this system, we have systematically analysed cell response in terms of force transmission, rearward flow and integrin recruitment after varying (i) gel rigidity, (ii) spacing and spatial distribution between RGD ligands, and (iii) integrin expression levels. Our results show that cell response and force generation are critically dependent on all factors. We also demonstrate the counter-intuitive fact that at specific ECM rigidities cells increase force transmission as the spacing between integrins increases from 50 to 100 nm. Our findings indicate that mechanical homeostasis can be tuned by cells using strategies based on integrin expression, clustering of ECM ligands, or ECM rigidity, and that an in-depth understanding of cell mechanical responses requires the consideration of all those factors.

Share this...

Facebook

Pinterest

Twitter

Linkedin