How does bacteria colonise space faster? What is the role of motility? To address this question, we grew E.coli colonies  in a semisolid matrix with nutrients (Fig. 1A) and imaged them using advanced microscope techniques (Fig. 1B). Some colonies presented a surprising morphology where an almost spherical colony (with a “bumpy” surface) was surrounded by smaller ones referred to as “satellites” (Fig. 1C).

We anticipate that  the formation of satellites is caused by single cells fleeing the mother colony to found and form new colonies. To test this theory, we embedded the bacteria in varying agarose concentrations. Increased stiffness reduced bacterial motility and hindered the formation of satellites. On the other hand, when the stiffness of the environment was reduced bacteria moved more freely and no longer formed compact aggregates. Satellites  appeared to be a transition state between completely confined cells and non-constraint ones.

But, why will cells detach from the main aggregate? When bacteria grow in communities they are more protected against external stresses than isolated cells. Numerical simulations (Fig. 2) provided insights to this question. A modified three-dimensional Eden growth model indicated  that colonies with satellites invaded the matrix at a faster rate than those without. Therefore, satellite formation is an advantageous trait if there is competition for resources.

We propose that satellites enable bacteria to colonise new territories quickly, exhibiting a super-linear spread over time while retaining the cohesive bindings characteristic of biofilms. Hence, satellite formation serves as a strategic advantage for bacteria, facilitating rapid invasion and permanent occupation in complex environments such as competing microbial communities, soils, or mammalian tissues.

Authors: Mireia Cordero, Namiko Mitarai and Liselotte Jauffred