Effectiveness ofPseudomonas aeruginosatype VI secretion system relies on toxin potency and type IV pili-dependent interaction

Abstract

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.

Cell motility greatly empowers bacterial contact weapons

“…S4). We observe that, similar to previous studies (5, 17, 35, 36), in the zero motility background there is a monotonic decrease in the efficacy of CDI as the inoculation density decreases and the initial clonal patches increase in size. At lower inoculum densities, the predominant mechanism by which motility improves the competitive advantage of the attacker is therefore genotypic mixing, which acts to disperse these patches.…”

“…S6D). Moreover, previous work with bacteria that lack surface motility also did not see this pattern, instead finding that higher inoculum density consistently enhances the efficiency of short-range weaponry (5, 17, 35, 36), likely due to the associated increase in initial intermixing of the two strains. These observations suggested that the non-monotonic relationship between inoculum density and CDI efficiency could be caused by changing effects of motility across densities.…”

“…This finding raises the possibility that the two mixing processes we describe here can be generalised across motility systems and species, as well as weapon systems. In support of this, recent work on the P. aeruginosa found that cell motility can improve killing via the T6SS (17).…”

“…This effect, which we call ‘genotypic mixing’ here, was expected to have a strong effect on the intoxification dynamics as the degree of genetic intermixing determines the inter-population interaction frequency, making it a key factor in social phenotypes (17, 20, 23). However, even when we begin the model with the two genotypes fully intermixed, we find that intoxification is substantially more efficient when motility is active compared to when it is not (Fig.…”

“…Consistent with this, these conditions are also closely associated with the expression of T6SS (13) and CDI genes (14). Moreover, deleting genes for motility systems can reduce the impact of contact weapons (15–17) suggesting that motility systems impact close-range combat.…”