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

The practicability of using the natural depuration method in Hong Kong for cleansing shellfish harvested before being sold for human consumption as routinely practised by some countries was examined. The method was based on the assumption that faecally-contaminated shellfish would quickly release the accumulated microorganisms into the surrounding water when they were placed 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 bacteriological quality mussels could be obtained from a rearing site with good water quality. The site chosen for the present study had comparatively good bacteriological water quality (with an annual geometric mean E. coli density of 12 per 100 ml). To the contrary, mussels harvested there were often found to contain many times more E. coli (average 2×103 E. coli per 100 g flesh) than that in the water. In vitro study by immersing UV-depurated clean mussels (containing less than 5 E. coli per 100 g flesh) in sterile seawater inoculated with high concentrations of E. coli (2.6×103 and 2.9×105 per 100 ml, 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 100 fold as much as that in the seawater; (c) levels of E. coli accumulated subsequently declined, first sharply, then at a slower rate within the next 15 to 20 h to approximately 0.2% of the peak level at the end of the experiment; (d) a slight uptake of E. coli was again observed in all experiments at around 25 to 30 and 40 to 45 h; and (e) the level of E. coli in the mussels remained more or less the same thereafter. The in vitro study results suggested that once shellfish had accumulated a high level of microorganisms, natural depuration or purification (by means of relaying contaminated shellfish in a body of clean water for some time before sale) might not necessarily be a reliable method to achieve an acceptable microbiological quality of shellfish for human consumption. In view of the local environmental water quality, cleansing shellfish by purification in the natural environment may not be practical in Hong Kong.