Active bulging promotes biofilm formation in a bacterial swarm

The interactions between microbes and their environment influence bacterial transport and colonization. Here we studied cell-fluid interaction in bacterial colonies and the consequence of such interaction on biofilm development. We found that an originally uniform swarming colony may spontaneously separate into different phases due to cell-fluid interaction, via a process that we called 'active bulging'. Using genetically encoded fluorescent reporters, we show that the active bulging process promotes biofilm formation. Our findings enrich the understanding of biofilm formation mechanisms and may inform the manipulation of self-assembly in bacterial active fluids.