  Contact between particles and motile cells underpins a wide variety of
biological processes, from nutrient capture and ligand binding, to grazing,
viral infection and cell-cell communication. The window of opportunity for
these interactions is ultimately determined by the physical mechanism that
enables proximity and governs the contact time. Jeanneret et al. (Nat. Comm. 7:
12518, 2016) reported recently that for the biflagellate microalga
Chlamydomonas reinhardtii contact with microparticles is controlled by events
in which the object is entrained by the swimmer over large distances. However,
neither the universality of this interaction mechanism nor its physical origins
are currently understood. Here we show that particle entrainment is indeed a
generic feature for microorganisms either pushed or pulled by flagella. By
combining experiments, simulations and analytical modelling we reveal that
entrainment length, and therefore contact time, can be understood within the
framework of Taylor dispersion as a competition between advection by the no
slip surface of the cell body and microparticle diffusion. The existence of an
optimal tracer size is predicted theoretically, and observed experimentally for
C. reinhardtii. Spatial organisation of flagella, swimming speed, swimmer and
tracer size influence entrainment features and provide different trade-offs
that may be tuned to optimise microbial interactions like predation and
infection.
