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.