Migration and settlement abilities are important biological traits to get advantageous living spaces, especially in spatially structured environments. In this study, the contribution of these traits to propagation under structured environments was empirically evaluated by using microcosms of a bacterium Pseudomonas aeruginosa. In our static culture system, P. aeruginosa cells migrate to oxygen-rich air-liquid interfaces by motility and settle down there by constructing a multicellular structure, known as a pellicle. As pellicle formation facilitates the growth or competitiveness, it is a selectively advantageous behavior of bacteria specific to static environments. In this work, we investigated the role of motility and settlement traits in pellicle formation process.

Various mutant strains were constructed by disruption of the genes related to motility or production of extracellular polymeric substances (EPS) and their pellicle formation properties were investigated. As a result, defect of motility (flagella-driven swimming) affected the pellicle formation rate and the pellicle morphology. The EPS-deficient strain exhibited significant delay in pellicle formation. These results indicate that both motility and settlement ability are involved in the proper construction of multicellular structures and acquiring the advantageous niches at the air-liquid interface.