Abstract

A microliter droplet of bacteria can grow and spread into a centimeter-sized colony on an agar gel surface within several hours, forming a variety of patterns. An expanding bacterial colony is often referred to as a swarm, indicating collective motion of the constituent bacteria that are individually motile. A bacterial swarm is an active fluid with rapid increase in total particle number and collective motion, but the swarm expansion is limited by water availability and surface tension. The strong coupling between the activity of individually motile bacteria and their surrounding fluid flow leads to rich pattern dynamics involving many coupled physical parameters. We study the swarming dynamics and pattern evolution following either conventional dot inoculation, a line inoculation, or an annular inoculation of Psuedomonas aeruginosa on the gel surface. With slight changes in agar percentage, ambient humidity, temperature, or level of surfactants, the dot inoculation is known to produce a rich variety of patterns. The ring inoculation leads to observation of edge-directed accumulation, wavelike structures due to hyper-elastic buckling, droplet formation due to the Rayleigh–Plateau instability, and collective migration of droplets and their coalescence in subsequent growth. Our experiments offer strong evidence that physical effects largely account for most patterns that develop in expanding bacterial colonies.