Toxigenic Vibrio baselines and optimum storage, transport and shelf-life conditions to inform cold supply chains in the north Australian Tropical Rock Oyster industry

Internationally, tropical rock oysters (TROs) have a poor safety reputation and a pro-active rather than reactive approach to Vibrio food safety is essential for product assurance and branding. Further, risk assessment needs to be informed by real data to ensure appropriate and proportional responses. There are certainly knowledge gaps for north Australia, but from previous work, seawater in Darwin Harbour was shown to contain up to 42 Vibrio spp. including several known toxigenic species in addition to the human pathogens Vibrio parahaemolyticus and V. vulnificus. Studies in the seasonal tropics (Darwin Harbour) showed that V. parahaemolyticus responds to temperature, despite being in the tropics where seawater temperatures are consistently high. V. vulnificus concentrations in seawater are higher in the wet season compared to the dry, and more shellfish are positive for V. parahaemolyticus and V. vulnificus in the wet season compared to the dry season. So if Vibrio diversity and abundance in TRO is seasonal (as shown elsewhere), it is likely that Vibrio spp. infections in humans will also follow a seasonal trend which has implications for risk management. A major bottleneck is that we do not know how vibrio numbers are affected by storage and transport temperatures in TRO. Studies have shown that Pacific and Sydney Rock Oysters have different vibrio growth curves for example, so it is not one size fits all and it is probable TROs will be different again.  In addition to identifying Vibrio baselines in TRO and developing tests for toxigenic species, we will identify the best post-harvest storage and transport temperatures and assess TRO shelf life at realistic storage temperatures. This will provide fundamental information to inform cold supply chains that will support farmers, wholesalers and retailers of TROs from northern Australia. We will also use this information to prepare an appropriate and regionally relevant Vibrio risk profile for TRO in northern Australia to assist initial risk management activities. The work described in this report will provide the developing TRO industry with the knowledge needed to make informed decisions about Vibrio in particular, and food risk more generally, and help ensure an exemplary reputation with access to premium markets.