Outer membrane vesicles containing signalling molecules and active hydrolytic enzymes released by a coral pathogen Vibrio shilonii AK1.

Production and release of outer-membrane vesicles (OMVs) is known in many bacteria including human pathogens. To date, OMV release has not been reported in coral-associated bacteria. We discovered that Vibrio shilonii AK1, a well-studied coral pathogen, produces OMVs in culture. Transmission electron microscopy showed that V. shilonii cultures release two types of vesicles, with a single membrane or two membranes, as well as vesicle chain-like morphotype in purified vesicle fraction. No significant difference was observed in the amount of OMVs produced by cultures grown at 20°C or 30°C. OMV proteomic analysis, never before done in a coral isolate, showed that a large number of low abundance proteins were exclusively detected in OMVs released by 20°C cultures. Further, the OMVs purified from AK1 cultures grown at both 20°C and 30°C carry N-acylhomoserine lactone quorum sensing signals, as well as alkaline phosphatase, lipase and chitinase activities. Our results show that V. shilonii OMVs are conduits of signalling molecules, active enzymes and other proteins to its environment. These findings suggest important ecophysiological roles of OMVs in coral reef environment. We discuss the importance of OMV release for V. shilonii fitness and propose several hypotheses as well as a conceptual model.