Substrate stiffness regulates collective colony expansion of the social bacterium Myxococcus xanthus

Many cellular functions depend on the physical properties of the cells environment. Many bacteria have different types of surface appendages to enable adhesion and motion on a variety of surfaces. Myxococcus xanthus is a social soil bacterium with two distinctly regulated modes of surface motility, termed the social motility mode driven by type iv pili and the adventurous motility mode based on focal adhesion complexes. How bacteria sense different surfaces and subsequently coordinate their collective motion remains largely unclear. Using polyacrylamide hydrogels of tunable stiffness, we found that wild-type M. xanthus spreads faster on stiffer substrates. Here, we show using motility mutants that disrupt adventurous motility suppresses this substrate-stiffness response, suggesting focal-adhesion-based adventurous motility is substrate-stiffness dependent. We also show that modifying surface adhesion by the addition of adhesive ligands, chitosan, increases the amount of M. xanthus flairs, a characteristic feature of adventurous motility. Taken together, we hypothesize a central role of M. xanthus adventurous motility as a driving mechanism for surface and surface stiffness sensing.