Horizontal gene transfer can reshape bacterial warfare

Bacteria commonly use molecular weaponry to kill or inhibit competitors. Genes encoding many of these weapons can be transmitted horizontally, but the impact on bacterial warfare is not understood. Here, we develop an ecological model of bacterial warfare where there is potential for horizontal gene transfer. Our model predicts that weapon gene transfer from an attacker to a target strain is possible, but at a low rate such that transfer has a negligible impact on competition outcomes. We tested the model empirically using a transmissible plasmid encoding colicin E2, a potent antibacterial toxin produced by Escherichia coli. As predicted by the model, we find that toxin plasmid transfer is feasible during warfare, but the resulting transconjugants remain rare. However, exploring the model further reveals realistic conditions where transfer has major impacts. Specifically, the model predicts that whenever competing strains have access to some unique nutrients, transconjugants can proliferate and reach high abundances. In support of these predictions, short- and long-term experiments show that transconjugants can thrive when nutrient competition is relaxed. Our work shows how horizontal gene transfer can reshape bacterial warfare in a way that can benefit a weapon gene but not the bacteria that carries it.