A widely-occurring family of pore-forming effectors broadens the impact of the Serratia Type VI secretion system

The ability to compete with diverse competitors is essential for bacteria to succeed in microbial communities. A widespread strategy for inter-bacterial competition is the delivery of antibacterial toxins, or effector proteins, directly into rival cells using the Type VI secretion system (T6SS). Whilst a large number of broad-spectrum enzymatic T6SS effectors have been described, relatively few which form pores in target cell membranes have been reported. Here, we describe a widely-occurring new family of T6SS-dependent pore-forming effectors, exemplified by Ssp4 of Serratia marcescens Db10. We show in vitro that Ssp4 forms regulated pores that have higher selectivity for cations and use molecular dynamics simulations to support a high resolution structural model of a tetrameric membrane pore formed by Ssp4. Notably, Ssp4 displays a distinct ion selectivity, phylogenetic distribution and impact on intoxicated cells compared with Ssp6, the other cation-selective pore-forming toxin delivered by the same T6SS. Ssp4 is also active against a wider range of target species than Ssp6, highlighting that T6SS effectors are not always broad-spectrum. Finally, use of Tn-seq to identify Ssp4-resistant mutants reveals that a mucA mutant of Pseudomonas fluorescens, which overproduces extracellular polysaccharide, provides resistance to T6SS attacks. We conclude that possession of two distinct T6SS-dependent pore-forming toxins may be a common strategy to ensure effective de-energisation of closely- and distantly-related competitors.