Report No SR3553



Oct 1993


The assessment and control of the microbiological content of shellfish has always been problematic. A consistent relationship between the microbial content of shellfish and the overlying water has never been demonstrated due, at least in part, to the fact that as they feed shellfish accumulate micro-organisms in their gut. As a consequence, the EC Directive on the Quality of Waters for Shellfish Growth (79/923/EEC) stipulates a microbiological requirement of 300 faecal coliforms (FC) per 100 ml of shellfish flesh and intervalvular fluid(FIV).

Remedial measures to give compliance with this standard have been complicated by the observation that samples of mussels from the shores of sea lochs remote from sewage input can exceed this standard. Additionally it has been shown that mussels can degrade bacteria, including E. coli. As a consequence, the metabolic rate of the mussels at the time of sampling could influence the levels of contamination detected by subsequent analysis. Finally, the number of bacteria detected can be influenced by the method of analysis used and there is a continuing debate as to the correct approach to use.

The study described in this report was carried out by Clyde River Purification Board at the request of WRc. The objective of the study was to try to establish the basic principles controlling the relationship between the bacterial concentration found in shellfish and the overlying waters.

The approach taken involved studying the faecal coliform concentration in the FIV of mussels kept in tanks of artificial and natural seawater. As the studies developed, the mussels and seawater were treated in several ways to control experimental variables. To control the quality- of the water both artificial and pasteurised natural seawater were used. To provide consistency in the faecal coliform concentration of the mussel FIV the waters used were seeded with a culture of E. coli. Differing depuration procedures were used in an attempt to stabilise the metabolic rate of the mussels.

The studies confirmed that the relationship between FC concentrations in water and mussel FIV is complex. Although there was some evidence that a relationship could be established, the metabolic rate of the shellfish seemed to be an important consideration. The nutrient content of the water influenced this relationship. Where this was low there was evidence that mussels would metabolise the bacterial cells in their gut. Suggestions for further work are given.

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