Pathogenic Vibrios associated with Marine Invertebrates and Shellfish

Quicklinks: | Tail Fan Necrosis in Southern Rock Lobsters | The VBNC State |  Vibrios associated with Oysters | Vibrios and Oyster Processing and Handling |

Introduction

Pathogenic marine vibrios such as V. parahaemolyticus and V. vulnificus are significant human pathogens. Some strains of vibrios are also known to cause diseases eg. Shell disease in various crustaceans such as lonsters and prawns, but also bacterial diseases of shellfish. Enteric disease associated with these pathogens is caused by ingestion of contaminated seafood. These organisms are usually only a problem in warmer waters that allow the growth.

Much of the South Australian Commercial fishery is associated with warm “Mediterranean” waters capable of supporting growth of pathogenic marine vibrios. However, little is known about the potential for contamination of marine based foods, especially filter feeding animals such as oysters. Oysters have the potential to concentrate these bacteria and therefore may represent a significant public health risk. This risk may be complicated by the fact that vibrios enter a viable but non-culturable (VBNC) state at seawater temperatures below 15C. Consequently, there is significant industry interest in determining potential risk posed by contamination of seafoods (such as oysters) with V. parahaemolyticus and V. vulnificus.

Current Seafood Projects

Role of Marine Vibrios in development of Tail Fan Necrosis during Live Holding of Southern Rock Lobsters

Project Staff: Connor Thomas (Supervisor), Richard Musgrove (Co-supervisor), Damian May

Project Description: Southern Rock Lobsters that are live held by fishers, develop necrotic lesions on the uropods of the tail.  These lesions increase in size and severity and can cause significant disfigurement of the tail fan, often accompanied by inflammation. In severe cases, tail fan necrosis can lead to loss of most of the uropod.  The significant erosion and melanisation accompanying this syndrome makes affected lobsters unsaleable.  This project is designed to examine the role physical damage and subsequent uropod tissue infection that occurs during holding, on the development of tail fan necrosis (TFN).

Uropod erosion typical of Tail Fan Necrosis

Project Outcomes: TFN lesions are characterised by tail erosion.  The lesion surface is colonised by bacteria.  These bacteria form microcolonies on the lesion tissue surface.  Limited microbial invasion of uropod tissue by bacteria occurs as a result of growth of these bacteria. TFN lesions are dominated by Vibrio spp. Physical damage to uropod tissue alone is insufficient to initiate development of TFN. Artificial infection of intentionally damaged uropod tissue with a V. parahaemolyticus isolate from a TFN lesion, leads to development of lesions typical of TFN. TFN does not lead to systemic infection in affected Rock Lobsters and the lobster immune system does not respond to bacteria associated with the cause of TFN.  We hypothesise that the extent of TFN is limited by melanisation reactions at the site of infection.  This creates a barrier that limits extensive microbial invasion and development of systemic infection.  The effectiveness of the barrier is thought to be increased by the fact that there is limited opprtunity for access to soft tissues by the bacteria.

SEM image of microcolonies of bacteria associated with Tail Fan Necrosis

The Viable but Non-Cutlturable State of V. parahaemolyticus

Project Staff: Connor Thomas, Shih Hsun (Stephen) Chen (PhD Student).

Marine vibriosenter a viable but non-culturable (VBNC) state at low temperatures in seawater.  The relevance of this state to the bacteria is speculative, but is assumed that it allows the bacteria to overwinter until seawater temperatures increase to a point where these bacteria are able to grow.

This study aims to characterise the VBNC state of V. parahaemolyticus strains.  In particular, the project is designed to carefully characterise the physiological conditions that affect entry into the VBNC state and recovery from this state.  The project also aims to determine the role of a putative resuscitation factor protein encoded by these bacteria, in entry/recovery to/from the VBNC state. 

Past Seafoof Projects

A supply chain assessment of marine vibrios in oysters: prevalence, quantification and public health risk

Project leaders: Connor Thomas (Chief Investigator), Tom Madigan, Andrew Pointon

Other project contributors: Ken Lee, Michael Whillas

Aug 2005 - Aug 2006

Project Objectives and Technical Outputs

• Determine numbers of marine vibrios and total viable counts of bacteria associated with oysters harvested from SA waters, across the annual growing cycle
• Determine numbers of marine vibrios and total viable counts of bacteria present in seawater associated with SA oyster leases during the annual growing cycle
• Obtain unequivocal evidence for the presence of potentially pathogenic strains of Vibrio spp. associated with SA oysters, seawater associated with oyster leases and retail product
• Provide information about the effects of current harvest and post-harvest handling on numbers of pathogenic marine vibrios

Outcomes

• The South Australian Research Council (SAORC) will be provided with a detailed report that outlines the potential risk to public health from oysters that are contaminated with pathogenic marine vibrios. Where appropriate, relevant risk management options will be developed for consideration by industry.
• Information developed in controlled temperature and retail studies will be used to develop recommendations for harvest techniques
• A strategy and methodology for routine examination of pathogenic marine vibrios in oysters will be developed

Microbiological validation of current storage and transport temperatures for Pacific oyster industries in Australia

Project Leaders: Connor Thomas (Chief Investigator), Thomas Madigan

Other project Team Members: Andrew Pointon, Matthew Muggleton, Ken Lee, Michael Whillas, Evans Judd. 

Oct 2006 - Oct 2007 

Project Description:

For industry to export, current AQIS specified arrangements for storage of oysters destined for export must be clarified.  The current AQIS Export Control (Fish and Fish Products) Orders 2005 are seen as ambiguous and subject to interpretation.  One interpretation is that live oysters should be stored at <5°C unless it can be shown that alternative arrangements “minimises the growth of pathogens that could adversely affect the fitness for human consumption”.  This project seeks to validate alternative storage arrangements and, in light of recent fatal food-borne infections, examine the impact of seasonal harvesting effects, current storage, and transport arrangements on microbiological quality of whole oysters. 

Planned Outcomes:

• The principal outcome of this project will be to provide producers of Pacific oysters with fundamental information that enables production and export of high quality (live) oysters while meeting AQIS Export Orders (2005).
• Development of standard methods for safe storage and transport of oysters.
• Improve confidence of industry sectors in quality assurance programs designed to ensure production of high quality and microbiologically safe oysters.