Molluscs can concentrate pathogens from their surrounding waters making them potential reservoirs of disease. In the US it is estimated that molluscs contaminated with sewage have caused 4% of mollusc-related illnesses in the last 25 years. However a greater and increasing problem has been mollusc-related diseases from indigenous pathogens, mainly vibrios, which have caused 20% of illnesses and 99% of deaths from consumption of molluscs. The current method for monitoring shellfish is using the MPN method for enumerating the levels of Escherichia coli and faecal coliforms. Shellfish which do not meet the standards for direct human consumption are sent for depuration until they are within the limits. This paper determines whether this current method is adequate for indicating the numbers of potentially pathogenic vibrios and to what extent their depuration rates differ.
Mussel were set up in tanks and experimentally contaminated with Vibrio cholerae, Vibrio parahaemolyticus and E.coli. They were then depurated using ozone and samples were taken after 0, 5, 24 and 44 h. The abundances of each bacterium were enumerated using the MPN technique and presence of each Vibrio spp. was confirmed using PCR analysis of specific genetic markers.
The results were not overly surprising. E.coli abundances reduced dramatically by 95% within 5 h and continued to decrease to 0.2% after 44 h. V. cholerae abundances reduced by 92% after 24 h however then started to increase with a final abundance of 10% after 44 h. V. parahaemolyticus numbers decreased by 70% after 5 h, remained constant until 24 h and then reduced further to 15% after 44 h. It should also be noted that vibrio abundances were initially much lower than initail E.coli numbers.
The study strongly suggests that different bacteria are depurated at different rates and that vibrios are depurated much slower than E.coli, however I am not overly confident in the accuracy of the MPN method. Since E.coli are depurated so quickly they reach legal limits fast, allowing very short depuration times. This means there are still a lot of potentially pathogenic vibrios remaining in the shellfish which could start to proliferate again. There are currently no provisions for controlling or monitoring vibrios in shellfish traded within the EU. This is obviously not good news as vibrio abundances and incidents of disease seem to be increasing and this study strongly suggests that our current method for monitoring them does not work. New methods need to be developed to monitor vibrio abundances in shellfish, which will probably be incredibly difficult or alternative methods to depuration need to be adopted.
A review of: Croci, L., Suffredini, E., Cozzi, L. and Toti, L. (2002) Effects of depuration of molluscs experimentally contaminated with Escherichia coli, Vibrio cholerae 01 and Vibrio parahaemolyticus. Journal of applied microbiology. 92, 460-465.
3 comments:
Hi, I agree the rates they could replicate are alarming, meaning a batch that meets standers could rapidly pass this point during export, what would you suggest would be a new system to monitor Vibrio spp. ? Surely depuration pre sales would resolve the issue?
The problem isnt really them replicating during export it just that vibrios are depurated much slower then e.coli which are used to indicate them. So the e.coli limits can be reached really fast allowing shorter depuration times leaving high numers of vibrios in the mussels being sold which could then become greater if not stored correctly. There arent really any simple, good methods for detecting them atm without priorenrichment and even then you would probably need someway to differenciate between virulent and nonvirulent strains. The easier option would be to try to persuade people to cook shellfish .
I see thanks
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