  We present a coarse grained rigid multibody model of a subunit assembled
helix-like polymer, e.g., adhesion pili expressed by bacteria, that is capable
of describing the polymers force-extension response. With building blocks
representing individual subunits the model appropriately describes the complex
behavior of pili expressed by the gram-negative uropathogenic Escherichia coli
bacteria under the action of an external force. Numerical simulations show that
the dynamics of the model, which include both the effects of unwinding and
rewinding, are in good quantitative agreement with the characteristic
force-extension response as observed experimentally for type 1 and P pili. By
tuning the model, it is also possible to reproduce the force-extension response
in the presence of anti-shaft antibodies, which dramatically changes the
mechanical properties. Thus, the model and the results in this work give
enhanced understanding of how a pilus unwinds under action of external forces
and provide new perspective of the complex bacterial adhesion processes.
