|Abstract: ||Pseudomonas aeruginosa is a versatile and prevalent opportunistic pathogen. It encodes a large arsenal of pathogenicity factors, and secrets a plethora of proteins using specialised protein secretion systems. The type VI secretion system (T6SS) delivers proteins directly into neighbouring bacteria or eukaryotic cells using a mechanism homologous to the T4 bacteriophage tail spike.
Three T6SS are encoded on the P. aeruginosa genome. The study of the H1-T6SS has been facilitated by the fact it can be activated by the manipulation of the RetS/Gac/Rsm regulatory cascade by deletion of retS. However, the precise signals required for activation of this cascade, resulting in H1-T6SS activation, are unknown. This work investigates the role of subinhibitory concentrations of antibiotics in activating the system, and shows that kanamycin is able to induce production of core H1-T6SS components. This activation requires a functional Gac/Rsm cascade, but it is not known if this is due to direct signalling via the cascade, or due to a dominant effect of RsmA repression.
The H2-T6SS is characterized in this work. We highlight key differences between the H2-T6SS cluster in PAO1 and PA14, including the presence of additional core T6SS components and putative secreted effectors. A strain is generated in which expression of the PA14 H2-T6SS cluster can be activated and tightly controlled by arabinose inducible promoters. The activity of the promoters is confirmed by the H2-T6SS dependent secretion of Hcp2 specifically upon arabinose induction.
We further consider two putative H2-T6SS secreted substrates, VgrG14 and Rhs14. Production of these proteins is observed following arabinose induction, but their secretion is not detected. The Rhs14 protein is characterised, and its possible role as a H2-T6SS dependent effector is discussed. Finally, the H2-T6SS system in PA14 is shown to inhibit the internalisation of P. aeruginosa PA14, in contrast to the previously published observations of the H2-T6SS promoting internalisation of PAO1.|