Natural depuration of shellfish for human consumption: A note of caution

The practicability of using the natural depuration method in Hung Kong for cleansing shellfish harvested before being sold for human consumption as ro utinely practised by some countries was examined. The method was based on t he assumption that faecally-contaminated shellfish would quickly release th e accumulated microorganisms into the surrounding water when they were plac ed in a natural environment with clean water. Hence, we first monitored the level of Escherichia coli in mussels and their growing-water in Tap Mun of Hong Kong; for 12 consecutive months to confirm whether acceptable bacteri ological quality mussels could be obtained from a rearing site with good wa ter quality. The site chosen for the present study had comparatively good b acteriological water quality (with an annual geometric mean E. coil density of 12 per 100 mi). To the contrary, mussels harvested there were often fou nd to contain many rimes more E. toll (average 2 x 10(3) E. coli per 100 g flesh) than that in the water. In vitro study by immersing UV-depurated cle an mussels (containing less than 5 E, coli per 100 g flesh) in sterile seaw ater inoculated with high concentrations of E. coil (2.6 x 10(3) and 2.9 x 10(5) per 100 mi, respectively, in different experiments) revealed that: (a ) E. coli uptake by mussels commenced almost immediately; (b) levels of E. coli accumulated peaked with 3 to 5 h of exposure and reached about 50 to 1 00 fold as much as that in the seawater; (c) levels of E, coil accumulated subsequently declined, first sharply, then at ii slower rate within the nex t 15 to 20 h to approximately 0.2% of the peak level at the end of the expe riment; (d) a slight uptake of E. toil was again observed in all experiment s at around 25 to 30 and 40 to 35 hi and (e) the level of E, coli in the mu ssels remained more or less the same thereafter. The in vitro study results suggested that once shellfish had accumulated a high level of microorganis ms, natural depuration or purification (by means of relaying contaminated s hellfish in a body of clean water for some time before sale) might not nece ssarily be a reliable method to achieve an acceptable microbiological quali ty of shellfish for human consumption. In view of the local environmental w ater quality, cleansing shellfish by purification in the natural environmen t may not be practical in Hong Kong. (C) 2000 Elsevier Science Ltd, All rig hts reserved.