Using atomic force microscopy for physical virology: touching and manipulating single virus particles

Atomic force microscopy (AFM) employs a nanometer-scale tip mounted on a microcantilever to scan surfaces where virus particles have been captured. Beyond generating high-resolution images of individual virions in liquid, AFM offers unique capabilities: manipulation of single particles, investigation of their biomechanical properties, and real-time observation of assembly and disassembly processes, including genome release. This chapter begins by outlining fundamental aspects of virus adsorption and imaging, highlighting, among other factors, the influence of tip-convolution artifacts. These principles are applied to reveal the adsorption behavior of the TGEV coronavirus on surfaces. Subsequent sections detail approaches for probing TMV's mechanical properties through single-indentation experiments and mechanical fatigue protocols. In this section, the mechanical fatigue approach is also discussed when used on 2D arrays of viral coat proteins. The review also discusses how these mechanical techniques can trigger genome release in minute virus of mice (MVM), a process that can alternatively be induced by temperature, as happens in bacteriophage T7. Finally, the chapter illustrates how AFM can serve as a nanomanipulation tool to move individual viruses across surfaces and estimate their adhesion strength.