Bows and swords: why bacteria carry short and long-range weapons

Bacteria possess a diverse range of mechanisms for inhibiting competitors, including bacteriocins, tailocins, the type VI secretion system, and contact-dependent inhibition. Why bacteria have evolved such a wide array of weapon systems remains a mystery. Here we develop an agent-based model to compare short-range weapons that require cell-cell contact, with long-range weapons that rely on diffusion. Our models predict that contact weapons are useful when an attacking strain is outnumbered, facilitating invasion and establishment. By contrast, ranged weapons tend to only be effective when attackers are abundant. We test our predictions with the opportunistic pathogen Pseudomonas aeruginosa, which naturally carries multiple weapons, including contact-dependent inhibition (CDI) and diffusing tailocins. As predicted, short-range CDI functions better at low frequency, while long-range tailocins require high frequency and cell density to function effectively. Head-to-head competitions between the two weapon types further support our predictions: a tailocin attacker only defeats CDI when it is numerically dominant, but then we find it can be devastating. Finally, we show that the two weapons work well together when one strain employs both. We conclude that short and long-range weapons serve different functions and allow bacteria to fight both as individuals and as a group.