Microbial Flora of Pacific Oysters (Crassostrea gigas) Subjected to Ultraviolet-Irradiated Seawater

The ability of oysters to purge themselves of microbial contaminants was investigated by identifying the microorganisms retained by oysters after they have been subjected to ultraviolet (UV) light-treated seawater. A UV intensity of 960 muw per min per cm(2) reduced the microbial count of seawater from 263 to 13 per ml. The coliform multitube test (MPN) was reduced from a high of 17 to <0.18 per 100 ml. Over 75% of the microorganisms found in treated seawater were Acinetobacter/Moraxella, Vibrio/Pseudomonas type II, and Flavobacterium/Cytophaga. With the exception of coliforms, the microbial composition of oysters subjected to UV-treated seawater remained at levels comparable to the control oysters held in untreated seawater. Total counts ranged between 10(3) and 10(5)/g. The microorganism most frequently encountered were Flavobacterium/Cytophaga, Vibrio/Pseudomonas type II, Pseudomonas type III or IV, Acinetobacter/Moraxella, gram-positive cocci and Bacillus. Together they comprised over 90% of the flora. Coagulase-positive, deoxyribonuclease-positive, and beta-hemolytic cocci were found in some samples, as were V. parahaemolyticus, V. aliginolyticus, and Aeromonas species.