  Viral fibers play a central role in many virus infection mechanisms since
they recognize the corresponding host and establish a mechanical link to its
surface. Specifically, bacteriophages have to anchor to bacteria through the
fibers surrounding the tail before starting the viral DNA translocation into
the host. The protein gene product (gp) 37 from bacteriophage T4 long tail
fibers forms a fibrous parallel homotrimer located at the distal end of the
long tail fibers. Biochemical data indicate that, at least three of these
fibers are required for initial host cell interaction, but do not reveal why
three and no other number. By using Atomic Force Microscopy we obtained
high-resolution images of gp37 fibers adsorbed on mica substrate in buffer
conditions and probed their local mechanical properties. Our experiments of
radial indentation at the nanometer scale provided a radial stiffness of ~0.08
N/m and a breaking force of ~120 pN. In addition, we performed Finite Element
Analysis and determined a Young's modulus of ~20 MPa. From these mechanical
parameters, we hypothesize that three viral fibers provide enough mechanical
strength to prevent a T4 virus from being detached from the bacteria by the
viral particle Brownian motion, delivering a biophysical justification of the
previous biochemical data.
