Optical Trapping of Individual Human Immunodeficiency Viruses in Culture Fluid Reveals Heterogeneity with Single-Molecule Resolution

Optical tweezers use the momentum of photons to trap and manipulate microscopic objects contact-free in three dimensions. Although this technique has been widely used in biology and nanotechnology to study molecular motors, biopolymers and nanostructures, its application in viruses has been very limited largely due to the small size of these nanoparticles. Using optical tweezers that can simultaneously resolve two-photon fluorescence at single-molecule level, here we show that individual HIV-1 can be optically trapped and manipulated, which allows multi-parameter analysis of single virions in culture fluid under native conditions. We show that individual HIV-1 differs in the numbers of envelope glycoproteins by more than one order of magnitude, which implies substantial heterogeneity of these virions in transmission and infection at single-particle level. Analogous to flow cytometry for cells, this fluid-based technique may allow ultrasensitive detection, multi-parameter analysis and sorting of viruses and other nanoparticles in biological fluid with single-molecule resolution.