MONITORING TOTAL BACTERIA AND VIBRIONACEAE IN EASTERN OYSTERS Crassostrea virginica EXPOSED TO VARIOUS WATER QUALITY STRESSES

Consumption of uncooked oysters is known to cause serious health conditions due to the bioaccumulation of contaminants, especially during the warmer months. As filter feeders, oysters ingest bacteria along with their food sources, primarily phytoplankton, from their surrounding habitats. Ensuring seafood safe for human consumption is always a concern, since oysters can be consumed raw, and disease causing organisms, environmental contaminants, chemicals, toxins, and even physical hazards such as glass, wood, soil, and metals can enter thorough feeding and be retained in the oysters.

One of the biggest concerns with consumption of raw oysters is the risk of the bacteria, Vibrionecae. Contracting these bacteria through consumption of raw oysters can lead to fever, chills, nausea, vomiting, diarrhea, shock, skin lesions and in severe cases can lead to death. In this study, Eastern oysters (Crassostrea virginica) were exposed to various environmental stresses including high nitrate, low salinity, low and high suspended silt and clay in water in a controlled laboratory setting. The colonization of naturally occurring total bacteria and Vibrionaceae in Eastern oysters were compared to the control oysters and relationships between the environmental parameters and total aerobic bacteria and Vibrionecae levels were investigated. The five trials, approximately 300 Eastern oysters from the Broadkill River in Lewes, Delaware were used, 60 oysters for each trial.  Baseline Vibrio levels in the oysters were detected using the Colony Overlay Procedure for Peptidases (COPP) assay prior to the treatments.  The remaining two hundred forty oysters were randomly placed into 16 - 2 L sterile containers filled with 1.5 L of 1 µm filtered seawater, each housing three to four oysters. Of the total 16 containers; three containers included a low salinity treatment of 10 - 15 ppt seawater, three containers included a salinity of 20 - 25 ppt seawater and referred as a control, three containers had a high nitrate concentration (0.30 mg/L) via addition of 0.3 g of potassium nitrate (KNO₃), and the last six containers included  high suspended solid treatment of 1,000 mg/L and a low suspended solid treatment of 100 mg/L prepared by using the settled silt particles and organic material from the Broadkill River.

Correlations between water quality, total aerobic bacteria, and Vibrionaceae were examined. Within the control treatments, nitrite and total suspended solids were strongly related to total aerobic bacteria (R² = 99.9% and 92.9%, respectively), while total nitrogen and total reactive phosphorus were strongly related to Vibrionaceae (R² = 93.1% and 92.8%, respectively).  The increased nitrate treatments showed a strong relationship between total aerobic bacteria and nitrate and nitrite (R² = 99.4% and 99.9%, respectively), whereas Vibrionaceae was strongly related to total nitrogen and total reactive phosphorus (R² = 94.6% and 79.3%, respectively).  Strong relationships were observed between total aerobic bacteria and nitrate and total suspended solids (R² = 90.5% and 92.5%, respectively) and between Vibrionaceae and total suspended solids (R² = 89.0%) in the low salinity treatments.  Low salinity treatment oysters consistently yielded the highest total aerobic bacteria and Vibrionaceae levels, followed by the high nitrate treatment oysters, the high average total suspended solids treatment oysters, and the control treatment oysters. Although the salinity treatments had the highest total aerobic bacteria and Vibrionaceae levels, the data was weakly related to the nutrients. Therefore, this study confirms the high aerobic bacteria and Vibrionaceae levels are primarily salinity dependent.