Microbial adhesion and ultrastructure from the single-molecule to the single-cell levels by Atomic Force Microscopy

In the last decades, atomic force microscopy (AFM) has evolved towards an accurate and lasting tool to study the surface of living cells in physiological conditions. Through imaging, single-molecule force spectroscopy and single-cell force spectroscopy modes, AFM allows to decipher at multiple scales the morphology and the molecular interactions taking place at the cell surface. Applied to microbiology, these approaches have been used to elucidate biophysical properties of biomolecules and to directly link the molecular structures to their function. In this review, we describe the main methods developed for AFM-based microbial surface analysis that we illustrate with examples of molecular mechanisms unravelled with unprecedented resolution.

Probing Single Virus Binding Sites on Living Mammalian Cells Using AFM

In the last years, atomic force microscopy (AFM)-based approaches have evolved into a powerful multiparametric tool that allows biological samples ranging from single receptors to membranes and tissues to be probed. Force-distance curve-based AFM (FD-based AFM) nowadays enables to image living cells at high resolution and simultaneously localize and characterize specific ligand-receptor binding events. In this chapter, we present how FD-based AFM permits to investigate virus binding to living mammalian cells and quantify the kinetic and thermodynamic parameters that describe the free-energy landscape of the single virus-receptor-mediated binding. Using a model virus, we probed the specific interaction with cells expressing its cognate receptor and measured the affinity of the interaction. Furthermore, we observed that the virus rapidly established specific multivalent interactions and found that each bond formed in sequence strengthens the attachment of the virus to the cell.

DNA building blocks for AFM tip functionalization: An easy, fast and stable strategy

Biosensing atomic force microscopy (AFM) offers the unique feature to determine the energy landscape of a bimolecular interaction at the real single molecule level. Furthermore, simultaneous and label-free mapping of molecular recognition and the determination of sample topography at the nanoscale gets possible. A prerequisite and one of the major parts in biosensing AFM are the bio-functionalized AFM tips. In the past decades, different approaches for tip functionalization have been developed. Using these functionalization strategies, several biological highly relevant interactions at the single molecule level have been explored. For the most common approach, the use of a heterobifunctional poly(ethylenglycol) crosslinker, a broad range of linkers for different chemical coupling strategies is available. Nonetheless, the time consuming functionalization protocol as well as the broad distribution of rupture length reduces the possibility of automation and may reduce the accuracy of the results. Here we present a stable and fast forward approach based on tetra-functional DNA tetrahedra. A fast functionalization and a sharp defined distribution of rupture length gets possible with low effort and high success rate. We tested the performance on the classical avidin biotin system by using tetrahedra with three disulfide legs for stable and site directed coupling to gold coated tips and a biotinylated end at the fourth vertex. A special advantage appears when working with a DNA aptamer as sensing molecule. In this case, the fourth strand can be extended by a certain DNA sequence complementary to the linkage part of an aptamer. This AFM tip functionalization protocol was applied on thrombin using DNA aptamers directed against the fibrinogen binding side of human thrombin.

Molecular Recognition Force Spectroscopy for Probing Cell Targeted Nanoparticles In Vitro

In the development and design of cell targeted nanoparticle-based systems the density of targeting moieties plays a fundamental role in allowing maximal cell-specific interaction. Here, we describe the use of molecular recognition force spectroscopy as a valuable tool for the characterization and optimization of targeted nanoparticles toward attaining cell-specific interaction. By tailoring the density of targeting moieties at the nanoparticle surface, one can correlate the unbinding event probability between nanoparticles tethered to an atomic force microscopy tip and cells to the nanoparticle vectoring capacity. This novel approach allows for a rapid and cost-effective design of targeted nanomedicines reducing the need for long and tedious in vitro tests.