Effectiveness of Pseudomonas aeruginosa type VI secretion system relies on toxin potency and type IV pili-dependent interaction

Author summaryThe Type VI Secretion System (T6SS) was discovered in Gram-negative bacteria and is a contact-dependent molecular nanomachine which injects antimicrobial toxins into competitors. The T6SS activity provides a competitive edge for an organism to prevail in a given niche. The number and biochemical activity of toxins injected could vary from one bacterial species to the other. The opportunistic pathogen Pseudomonas aeruginosa is quite prolific with more than 20 characterized and distinct T6SS toxins. Here we experimentally addressed the importance in the degree of T6SS activity and made the demonstration that every single of the many P. aeruginosa T6SS toxin counts. There are no redundancies and instead on occasion synergies, which support the effectiveness of injecting a cocktail of toxins rather than a subset of specific ones. Since contact is a key factor for T6SS delivery, we further observed that preys able to use surface motility skills to run away from bacterial attackers have better chances to multiply and survive. Furthering experimental approaches, we used computational modelling to place these data in the context of a mixed P. aeruginosa population. This way we contribute understanding to how highly local contact-based interactions between individuals shape the structure of whole bacterial communities. The type VI secretion system (T6SS) is an antibacterial weapon that is used by numerous Gram-negative bacteria to gain competitive advantage by injecting toxins into adjacent prey cells. Predicting the outcome of a T6SS-dependent competition is not only reliant on presence-absence of the system but instead involves a multiplicity of factors. Pseudomonas aeruginosa possesses 3 distinct T6SSs and a set of more than 20 toxic effectors with diverse functions including disruption of cell wall integrity, degradation of nucleic acids or metabolic impairment. We generated a comprehensive collection of mutants with various degrees of T6SS activity and/or sensitivity to each individual T6SS toxin. By imaging whole mixed bacterial macrocolonies, we then investigated how these P. aeruginosa strains gain a competitive edge in multiple attacker/prey combinations. We observed that the potency of single T6SS toxin varies significantly from one another as measured by monitoring the community structure, with some toxins acting better in synergy or requiring a higher payload. Remarkably the degree of intermixing between preys and attackers is also key to the competition outcome and is driven by the frequency of contact as well as the ability of the prey to move away from the attacker using type IV pili-dependent twitching motility. Finally, we implemented a computational model to better understand how changes in T6SS firing behaviours or cell-cell contacts lead to population level competitive advantages, thus providing conceptual insight applicable to all types of contact-based competition.