Cell motility greatly empowers bacterial contact weapons

Many bacteria kill competitors using short-range weapons, such as the Type VI Secretion System (T6SS) and Contact Dependent Inhibition (CDI). While these can deliver powerful toxins, they rely on direct contact between attacker and target cells. We hypothesised that movement enables attackers to contact more targets and thus greatly empower their weapons. To explore this, we developed individual-based and continuum models to show that motility greatly improves contact-dependent toxin delivery through two underlying processes. First, genotypic mixing increases the inter-strain contact probability of attacker and sensitive cells. Second, target switching ensures attackers constantly attack new cells, instead of repeatedly hitting the same cell. We test our predictions with the pathogen Pseudomonas aeruginosa, using genetically engineered strains to study the interaction between CDI and twitching motility. As predicted, we find that motility massively improves the effectiveness of CDI, in some cases up to 10,000-fold. Moreover, we demonstrate that both mixing processes occur using timelapse single-cell microscopy and quantify their relative importance by combining experimental data with our models. Our work shows how bacteria combine cell movement with contact-based weapons to launch powerful attacks on their competitors. ### Competing Interest Statement The authors have declared no competing interest.