  Biological microswimmers such as bacteria and sperm cells often encounter
complex biological fluid environments. Here we use the well-known squirmer
microswimmer model to show the importance of the local fluid microstructure and
non-continuum effects on their swimming speed in different polymeric and
filamentous fluids. Surprisingly, we find that different squirmer types move at
considerably different speed in filamentous fluids which cannot be explained by
existing continuum models, but by considering the local fluid and polymer
properties around the squirmers. Furthermore, direct squirmer-polymer
interactions slow down in particular pushers by trapping large stiff filaments
in a self-generated recirculation region in front of them.
