Scientific Alert Note

A new fast method has been developed for detecting pathogenic bacteria in seafood

Scanning electron micrograph of Vibrio parahaemolyticus with characteristic filamentous flagellae

At present there are no pan-European requirements to monitor for V. parahaemolyticus in either harvesting areas or ready-to-eat seafood, nor are there formal epidemiological surveillance systems. The absence of EU-wide legislative controls is largely due to the widely acknowledged lack of suitable methods that allow differentiation of the pathogenic strains of the bacterium from those that are not considered a human health risk. This however, has not prevented several European countries introducing unilateral controls for imported products. In such cases the use of inappropriate test methods can potentially lead to a significant number of border rejections of imported seafood that may not constitute a public health risk. Consequently, the main aim of the SEABAC project was to establish suitable methods based upon novel molecular technologies that will enable the prevalence of pathogenic strains in European seafood to be established and as a result estimate the real risk for seafood consumers. The SEABAC project includes six partners from research institutes and universities in Spain, Portugal, Italy, Iceland, France and the U.K.

 

 

 Author: Rachel Rangdale, CEFAS, UK

 

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SEABAC project group members outside the Icelandic Fisheries Institute, Reykjavik

Vibrio parahaemolyticus is a common marine bacterium found in tropical and warmer temperate seas. Consumption of seafood contaminated with V. parahaemolyticus can lead to acute gastro-enteritis. This is because a minority of strains produce thermostable directly or directly related haemolysins which are pathogenic in man. On average in the U.S. around 3,000 cases per year are recorded and in South East Asia, a pandemic serotype of V. parahaemolyticus (O3:K6) is responsible for around 40% of all seafood-associated illness.

 

Historically, in Europe, cases have been rare and usually related to travel to endemic regions or cross-contamination of seafood following harvest. However, recently several outbreaks have been reported in Europe associated with indigenously produced seafood. In 2004, O3:K6, V. parahaemolyticus-associated gastroenteritis was reported in Spain. Investigation of the outbreak implicated the consumption of edible crabs (Cancer pagurus) that had originated in the U.K. It was suggested, but not proven, that improper post harvest handling was the cause of contamination.  Molecular characterisation of isolates collected from patients during this outbreak showed the emergence of the O3:K6 pandemic strain in Europe. Recently, this pandemic strain has been identified in the Northern Adriatric Sea indicating that it may be becoming established in European waters. It has been speculated that this increased incidence may be associated with warmer sea temperatures attributed to climate change.

 

To date, over 100 strains originating from various seafood and patients have been collected as part of the SEABAC project.  These have been used to help develop real-time PCR and nucleic acid hybridisation methods that enable detection of pathogenic strains in seafood. Molecular characterisation tools such as Pulse Field Gel Electrophoresis (PFGE) (below) that enable examination of the entire bacterial genome have been used to compare European seafood derived strains with those have been collected from patients in Europe, South East Asia and elsewhere. PFGE fingerprints patterns have shown that strains from the environment are closely related to each other but show little similarity with clinically significant strains such as the pandemic clone O3:K6. 

 

Methods developed within the project have also been used to test samples of bivalve shellfish and various species of crustacean. Results have indicated that although V. parahaemolyticus strains are present in a significant number of samples (e.g. approximately 30% of samples tested in the UK) the proportion of strains possessing the thermostable haemolysins and thus able to cause disease is low.  This data highlights the importance of the use of standardised methods for the detection and characterisation of bacterial species in seafood across the EU to accurately assess risk. SEABAC methods are being adopted as official standards at CEN (European Normalisation Committee) and may in time form the basis of new EU wide food hygiene regulations for pathogenic vibrios. The integration and harmonisation of these methods on a pan European level will advance the knowledge base within the EU and provide important information on future decisions on controls.