Cytosol-invading Salmonella enterica serovar Typhimurium are labelled with a ubiquitin coat, which serves the dual purpose of targeting bacteria to autophagy-mediated degradation and locally inducing pro-inflammatory signalling. Importantly, the chain types and ubiquitylated substrates that constitute this ubiquitin coat are still poorly characterised. Here, I show that the ubiquitin coat of Salmonella is highly dynamic and undergoes significant compositional changes over the course of an infection. I also identify a novel ubiquitylated substrate on the bacterial surface.

In contrast to Salmonella Typhimurium, a bacterium ill-adapted to cytosolic survival, Shigella flexneri is a professional cytosol-dwelling pathogen and evades targeting by host E3 ligases. In this work, I developed SPLINT Labelling, a novel tool to artificially induce ubiquitylation of S. flexneri by enforcing the recruitment of a Salmonella-targeting E3 ligase to the surface of Shigella. I demonstrate that SPLINT Labelling with LUBAC induces a M1-linked ubiquitin coat on S. flexneri and mediates the recruitment of the downstream antibacterial effectors P62, OPTINEURIN, NEMO and LC3. Nevertheless, SPLINT Labelling fails to inhibit proliferation of cytosolic S. flexneri.

Overall, this work further defines the role of ubiquitylation for the detection and restriction of two different cytosol-invading pathogens.