Report No SR 97(04)F

Investigation of the relationship between indicator bacteria in mussel flesh and intervalvular fluid and surrounding waters Phase 2

SR 97(04)F

Dec 1997

EXECUTIVE SUMMARY

  1. The EC Directive on the Quality of Waters for Shellfish Growth (79/923/EEC) stipulates a microbiological requirement of 300 faecal coliforms per 100ml of shellfish flesh and intervalvular fluid. After two years of sampling, it emerged, from results collated from all over the UK, that mussels collected from sites remote from any notable sewage input were consistently exceeding this standard. Archives revealed that there was a great dearth of research in this area; therefore it was decided, in 1992, to fund a preliminary investigation (report SR3553) into the relationship between FC levels in mussel FIV and their surrounding waters. It soon became clear that there was no empirical relationship and a further study (report SR[97]03F) revealed a complex relationship leading to the investigation of an alternative indicator bacterium. Faecal streptococci was found to be a promising candidate, and, this study continues the development of FS as an indicator of faecal pollution in mussel FIV.
  2. The objectives of this study, which follows on from report SR(97)03F were, i) to monitor FC and FS levels in mussel FIV during comparative depuration experiments, with varying salinity, ii) to monitor FC and FS levels in mussels which have been relayed from highly polluted waters to non-polluted waters, iii) to monitor FC and FS levels in mussels and their adjacent waters employing local sampling sites and nation-wide sampling sites and iv) to further develop the FS pour plate method for the quantitative analysis of mussel FIV.
  3. At the end of each 10 day depuration experiment FIV FS levels were always lower than FIV FC levels. The final FIV FC levels were always >300/100ml; whereas the final FIV FS levels were always < 200/100ml and the majority of results were <100/100ml. This suggests that FIV FS levels are a better indicator of faecal pollution when mussels are growing in relatively unpolluted waters.
  4. When mussels, which were collected from a polluted area, were relayed in an area of virtually no human faecal pollution, FIV FC levels from both the tidal and sub-tidal zones did not achieve the mandatory standard, of 300/100ml, as cited in the EC Directive on the Quality of Waters for Shellfish Growth (79/923 EEC). In contrast the FIV FS levels from the tidal and sub-tidal zones were <100/100ml. This is further evidence that FIV FS levels may be a suitable indicator of faecal pollution when mussels are growing in relatively unpolluted waters.
  5. There appears to be a very strong linear relationship between FIV FS and FS in their adjacent waters, when analysis is carried out within six hours of sampling. The pour plate method displays a higher correlation coefficient than the MPN method; therefore, not only is the pour plate method quicker and cheaper, but, it may also be more reproducible.
  6. There appears to be no relationship between FIV FC and FC in their adjacent waters, when analysis is carried out within six hours of sampling. This confirms the findings in previous studies.
  7. A powerful case is emerging to replace FIV FC levels with FIV FS levels as the major microbiological parameter in EC Directive on the Quality of Waters for Shellfish Growth (79/923/EEC). This case should be further developed in future studies.
  8. As part of the programme to develop the FS mussel / water relationship, mussels were collected from all over the UK, transported overnight to the SEPA West Microbiology Laboratory and analysed approx 24 hours after sampling. It soon became apparent t hat the data from the transported samples did not fit the developed FS mussel / water relationship. Therefore, it is a possibility that FS concentrations in mussels differ significantly 24 hours after compared to 6 hours after sampling . As it is common practice, all over the UK, for procedures relating to mussel sampling and analysis to include up to a 24 hour delay when sampling sites are remote from the laboratory, this phenomenon should be examined in greater detail in the next phase of the study.

KEY WORDS

Mussels, Mytilus edulis, artificial sea water, Faecal coliforms, Faecal streptococci, pour plates, most probable number, salinity, organic nutrients, adjacent waters, depuration.

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