Accumulation, Persistence, and Elimination of Microbial Indicators by Molluscan Shellfish

The accumulation of fecal coliforms and several other indicator microorganisms (Escherichia coli, enterococci, Clostridium perfringens, and male-specific bacteriophage) by hard-shelled clams (Mercenaria mercenaria) was investigated with regard to season and temperature. Very high rates of accumulation were found for each of these indicator organisms during distinct periods in the spring. Uptake of indicators during these periods exceeded 2 SD of the mean annual accumulation rates. Except for these periods of \u27hyper-accumulation\u27, uptake of the indicators is at least partially temperature dependent. In general, bacteriophage and C. perfringens were accumulated by clams at significantly higher rates than were either fecal coliforms or E. coli. Elimination of these microorganism from hard-shelled clams and oysters (Crassostrea virginica) using controlled purification procedures (depuration) was investigated under a variety of conditions in ultraviolet (UV) light disinfected seawater. C. perfringens and male-specific bacteriophage were considerably more refractory to this process than either fecal coliforms or enterococci. Trials using banded shellfish demonstrated that indicator elimination and not loss in viability primarily accounted for observed reductions in microbial indicator levels. Because of problems inherent to the UV disinfected systems (notably virus elimination), ozone was evaluated as an alternative means for disinfecting depuration waters. Ozone was found to be very effective for inactivating indicator organisms in seawater. However, the depuration rates of the same microorganisms from clams and oysters were not different from those observed with the UV disinfected systems. In fact, excessive amounts of ozone (³ 0.40 ppm) appear to significantly retard the rates of indicator elimination. The survival and replication of male-specific bacteriophage was investigated in hard-shelled clams and their homogenates and supernatants to further determine the potential use of these bacterial viruses as sanitary indicators. They were unable to replicate in vivo under ordinary conditions, although replication did occur in temperature abused shellfish homogenates and supernatants if a suitable host cell was present. Overall results from this study suggest that male-specific bacteriophage or C. perfringens may be better indices of the sanitary quality of molluscan shellfish than either fecal coliforms or E. coli