A type IVB secretion system adapted for bacterial killing, biofilm invasion and biocontrol

Many bacteria utilize contact-dependent killing machineries to eliminate rivals in their environmental niches. Here, we show that Pseudomonas putida IsoF is able to outcompete a wide range of bacteria with the aid of a novel type IVB secretion system (T4BSS) that can deliver toxic effectors into bacterial competitors. This extends the host range of T4BSSs, which were so far thought to transfer effectors only into eukaryotic cells, to prokaryotes. Bioinformatic and genetic analyses showed that this killing machine is entirely encoded by a rare genomic island, which has been recently acquired by horizontal gene transfer. IsoF utilizes this secretion system not only as a defensive weapon to antagonize bacterial competitors but also as an offensive weapon to invade existing biofilms, allowing the strain to persist in its natural environment. Furthermore, we show that IsoF can protect tomato plants against the plant pathogen Ralstonia solanacearum in a T4BSS-dependent manner, suggesting that IsoF capabilities can be exploited for pest control and sustainable agriculture.