Serratia marcescens Outer Membrane Vesicles rapidly paralyze Drosophila melanogaster through triggering apoptosis in the nervous system

The pathogenicity of Gram-negative bacteria is mediated by multiple virulence factors that likely include secreted Outer Membrane Vesicles (OMVs) that can act as a cargo for delivery of enzymes or toxins to target tissues. Here, we have studied the effects on the host of OMVs prepared from one of the most potent pathogens of Drosophila melanogaster , Serratia marcescens . OMV injection leads to the apparent demise of flies within few hours. We identify a number of host defenses that somewhat protect it from the action of OMVs, namely the systemic humoral immunity pathway Immune deficiency, Prophenol Oxidases 1&2, and the redox active enzymes Dual oxidase, NADPH-oxidase, and Nitric Oxygen Synthase. In contrast, unidentified hemocyte function(s) and the circulating protease Hayan promote the pathogenicity of OMVs. Mechanistically, we find that OMVs promote the activation of the JNK pathway and the transient expression of the pro-apoptotic genes head-involution defective and reaper in at least neurons. Our data suggest that mitochondrially-derived reactive oxygen species promote neuronal cell death that leads to the paralysis of OMV-injected flies. We identify the metalloprotease PrtA as a major virulence factor of OMVs and show that the injection of purified PrtA mimics most of the effects of OMVs. Finally, our data further indicate that PrtA contributes to the pathogenicity of injected Serratia marcescens .

This study underscores the potential for OMVs to act as virulence factors that efficiently target the nervous system in vivo despite the blood brain barrier.