Accumulation of dead cells from contact killing facilitates coexistence in bacterial biofilms

Bacteria often live in biofilms, crowded, surface-attached microbial consortia that typically feature a variety of species and strains. Social interactions between the different organisms govern the spatial organization of biofilms. These interactions can turn deadly, ultimately affecting biofilm composition. Bacteria have evolved deathly weapons that require killer cells to directly contact target cells, such as via the Type VI Secretion System (T6SS). The T6SS is hypothesized to be a highly potent weapon, capable of facilitating the invasion and defense of bacterial populations. However, we find that contact killing is self-limiting as dead cells accumulate. Time-lapse confocal microscopy of Vibrio cholerae strains that kill via the T6S shows that dead cell debris accumulates between competing strains, preventing physical contact and thus preventing killing. This renders contact killing V. cholerae incapable of invading or eliminating competitors on a community level. Instead, we find that contact killing can facilitate coexistence between nominally antagonistic strains. While a variety of defensive strategies against microbial warfare exist, the material consequences of cell death provide target cells with their first line of defense.