Microbiology of Commercial Depuration of the Sydney Rock Oyster, Crassostrea commercialis

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Abstract
Paired batches of unpurified and commercially purified oysters from a polluted estuary were examined for a range of indicator and pathogenic microorganisms on 16 occasions over a period of 1 year. Aerobic plate counts of purified oysters (geometric mean of all samples 4.8 × 102/g) were generally lower than those of unpurified oysters (geometric mean of all samples 1.2 × 103/g). Coliforms and Escherichia coli were detected considerably less frequently, and usually at lower levels in purified than in unpurified oysters. Three batches of purified oysters contained unacceptably high concentrations of E. coli. The purification process had little impact on the incidence or concentration of Vibrio parahaemolyticus, which was present at low levels (up to 48/g) in 12 batches of both unpurified and purified oysters. Non-01 serotypes of V. cholerae were also present in purified oysters. Viruses were not detected. In a subsequent survey, 54 oyster samples from 25 different purification plants and 5 estuaries were examined. Twenty one samples contained V. parahaemolyticus at levels up to 48/g and one contained V. cholerae (non-01). Ten samples contained E. coli. The results indicate that small numbers of potentially pathogenic vibrios are frequently a part of the microflora of purified oysters.