Trade-offs constrain adaptive pathways to the type VI secretion system survival.

The Type VI Secretion System (T6SS) is a nano-harpoon used by many bacteria to inject toxins into neighboring cells. While much is understood about mechanisms of T6SS-mediated toxicity, less is known about the ways that competitors can defend themselves against this attack, especially in the absence of their own T6SS. Here we subjected eight replicate populations of to T6SS attack by . Over ∼500 generations of competition, isolates of the populations evolved to survive T6SS attack an average of 27-fold better, through two convergently evolved pathways: was mutated in six of the eight replicate populations, while the other two populations each had mutations in both and . However, the mutations we identified are pleiotropic, reducing cellular growth rates, and increasing susceptibility to antibiotics and elevated pH. These trade-offs help us understand how the T6SS shapes the evolution of bacterial interactions.