Vibrio cholerae and enteric bacteria in oyster-producing areas of two urban estuaries in Australia.

Three sampling sites in oyster-producing areas of 2 estuaries were monitored at intervals of about 2 weeks for 1 year. Oysters (Crassostrea commercialis), water and sediment were examined for Vibrio cholerae, Escherichia coli and Salmonella. V. cholerae was detected in 20, 30 and 11% of oyster, water and sediment samples respectively. The highest incidence was in the autumn (March-May), with few isolations from July to October. Most isolates were non-O1 serotypes. The presence of V. cholerae and the enteric bacteria appeared to be influenced by different, but perhaps overlapping, sets of factors in these high salinity waters. There was no relationship between rainfall or salinity and the detection of V. cholerae, whereas high counts of E. coli in oysters and the presence of Salmonella were correlated wtih rainfall and, to a lesser degree, reduced salinity. High counts of E. coli were correlated with V. cholerae isolations from water and with the presence of Salmonella. Oysters concentrated E. coli effectively. The counts of E. coli in oysters were 7.3 times higher than those in water. Examination of 8 batches of purified and unpurified oysters indicated that purification reduces the incidence of V. cholerae. However V. cholerae was detected in 3 of 25 market samples of oysters, demonstrating that it can be present in oysters throughout the distribution system. The highest V. cholerae count observed in oysters was 3/g.