Droplet printing reveals the importance of micron-scale structure for bacterial ecology

Bacteria often live in diverse communities where the spatial arrangement of strains and species is considered critical for their ecology, including whether strains can coexist, which are ecologically dominant, and how productive they are as a community 1,2 . However, a test of the importance of spatial structure requires manipulation at the fine scales at which this structure naturally occurs 3–8 . Here we develop a droplet-based printing method to arrange different bacterial genotypes across a sub-millimetre array. We use this to test the importance of fine-scale spatial structure by printing strains of the gut bacterium Escherichia coli that naturally compete with one another using protein toxins 9,10 . This reveals that the spatial arrangement of bacterial genotypes is important for ecological outcomes. Toxin-producing strains largely eliminate susceptible non-producers when genotypes are well-mixed. However, printing strains side-by-side creates an ecological refuge such that susceptible strains can coexist with toxin producers, even to the extent that a susceptible strain outnumbers the toxin producer. Head-to-head competitions between toxin producers also reveals strong effects, where spatial structure can make the difference between one strain winning and mutual destruction. Finally, we print different potential barriers between two competing strains to understand why space is so important. This reveals the importance of processes that limit the free diffusion of molecules. Specifically, we show that cells closest to a toxin producer bind to and capture toxin molecules, which creates a refuge for their clonemates. Our work provides a new method to generate customised bacterial communities with defined spatial distributions, and reveals that micron-scale changes in these distributions can drive major shifts in their ecology.