A microbial hare and tortoise story: interactions between topological defects in biofilms favour bacteria that move more slowly

Motility is commonly observed in microbes, allowing them to explore their environment and exploit new resources. However, the possibility that increased motility can actually impede success has remained largely unexplored to date. We study Pseudomonas aeruginosa cells that move across solid surfaces using microscopic grappling hooks called pili. Surprisingly, we find that a mutant strain which individually moves faster than wild-type cells moves more slowly as a densely-packed group. Using cell tracking, active nematic theory, and simulations of self-propelled rods, we show that this phenomenon is caused by the behaviour of +1/2 topological defects within the collective. When two +1/2 defects composed of the faster moving cells collide, active forces generated by the cells cause the two defects to fuse into a single +1 defect. This spontaneously escapes into the third dimension, reorienting the faster moving cells vertically and so arresting their motion. The slower moving wild-type cells avoid this trapping mechanism and remain in the plane, allowing them to move unimpeded into new territory and ultimately outgrow the mutant. Our results show that the physics of active liquid crystals can have profound implications for the ecology and evolution of microbial communities.