Motility mediates satellite formation in confined biofilms

Bacteria have spectacular survival capabilities and can spread in many, vastly different environments. For instance, when pathogenic bacteria infect a host, the cells expand through proliferation and squeezing through narrow pores and elastic matrices. However, the exact role of surface structures and matrix elasticity in colony expansion and morphogenesis is still largely unknown. Here we show how satellite colonies emerge around biofilms embedded in semi-soft agar in controlled in vitro assays. We tested how extra-cellular structures - important for biofilm formation and motility - control this morphology. Moreover, we identify the range of extra-cellular matrix elasticity, where this morphology is possible. When paralleled with mathematical modelling, our results demonstrate that satellite formation allows bacterial communities to spread faster. We anticipate that this strategy is important to speed up expansion in various environments while retaining the close interactions and protection provided by the community.