Abstract:In an unshaken liquid culture, aerobic bacteria easily evolve to construct cellular mats at the air-liquid interface (ALI), a unique habitat with high oxygen access. Adaptive mutants typically overproduce extracellular matrices to form robust mat structures but grow slower than the ancestor. The genetic basis of mat formation in a model organism, Pseudomonas fluorescens, has been the subject of intensive investigation; however, little is known about the mechanical aspects that affect the emergence and maintenance of slow-growing mat formers. In this work, we revealed the diminished ALI colonization of the adaptive mutants due to the general trade-off between motility and matrix production. The deficiency in colonization and growth was overcome by in-situ mutations and low dispersal from the ALI. These findings proposed a novel evolutionary scenario in which adaptive mutants occupied a spatial niche not by migration but by in-situ mutations, highlighting the importance of pre-colonization by the ancestor and the population size. Given the ubiquity of the trade-off between motility and matrix production across bacteria, the mechanism revealed here is expected to be applicable to numerous environments, such as those affecting the adaptive evolution of crypt-colonizing bacteria in an animal gut.