  Actin filaments and microtubules both often form bundles of parallel
filaments within cells. Here, we shed light on a universal attractive
interaction between two such parallel filaments. Indeed, the electrodynamic
Casimir interaction between dielectric objects immersed in salted water at room
or body temperature includes a universal contribution that is unscreened by the
solvent and therefore long-ranged. We study this interaction between two
parallel cylinders immersed in salted water with strong Debye screening. We
show that its magnitude can largely exceed the energy scale of thermal
fluctuations in the case of actin filaments and microtubules in cells. While
multiple interactions exist between filaments in cells, this universal
attractive interaction should thus have an important role, e.g. in bundle
formation and cohesion.
