  The natural vibrational frequencies of biological particles such as viruses
and bacteria encode critical information about their mechanical and biological
states as they interact with their local environment and undergo structural
evolution. However, detecting and tracking these vibrations within a biological
context at the single particle level has remained elusive. In this study, we
track the vibrational motions of single, unlabeled virus particles under
ambient conditions using ultrafast spectroscopy. The ultrasonic spectrum of an
80-100 nm lentiviral pseudovirus reveals vibrational modes in the 19-22 GHz
range sensitive to virus morphology and 2-10 GHz modes with nanosecond
dephasing times reflecting viral envelope protein interactions. By tracking
virus trajectories over minutes, we observe acoustic mode coupling mediated by
the local environment. Single particle tracking allows capture of viral
disassembly through correlated mode softening and dephasing. The sensitivity,
high resolution, and speed of this approach promise deeper insights into
biological dynamics and early-stage diagnostics at the single microorganism
level.
