  Mucus is a biological gel covering the surface of several tissues and
insuring key biological functions, including as a protective barrier against
dehydration, pathogens penetration, or gastric acids. Mucus biological
functioning requires a finely tuned balance between solid-like and fluid-like
mechanical response, insured by reversible bonds between mucins, the
glycoproteins that form the gel. In living organisms, mucus is subject to
various kinds of mechanical stresses, e.g. due to osmosis, bacterial
penetration, coughing and gastric peristalsis. However, our knowledge of the
effects of stress on mucus is still rudimentary and mostly limited to
macroscopic rheological measurements, with no insight into the relevant
microscopic mechanisms. Here, we run mechanical tests simultaneously to
measurements of the microscopic dynamics of pig gastric mucus. Strikingly, we
find that a modest shear stress, within the macroscopic rheological linear
regime, dramatically enhances mucus reorganization at the microscopic level, as
signalled by a transient acceleration of the microscopic dynamics, by up to two
orders of magnitude. We rationalize these findings by proposing a simple yet
general model for the dynamics of physical gels under strain and validate its
assumptions through numerical simulations of spring networks. These results
shed new light on the rearrangement dynamics of mucus at the microscopic scale,
with potential implications in phenomena ranging from mucus clearance to
bacterial and drug penetration.
