Microbial Flora of Pacific Oysters (Crassostrea gigas) Subjected to Ultraviolet-Irradiated Seawater1

Abstract

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 ,gw per min per cm2 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 103 and 105/g. The microorganism most frequently encountered were Flavobacterium/Cytophaga, Vibrio/Pseudomonas type II, Pseudomonas type III or IV, AcinetobacterlMoraxella, 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.

Bacteria in bivalve shellfish with special reference to the oyster

“…but fewer micrococci and coryneforms, which are common soil bacteria. Similar observations were made on bivalves from the west coast of the United States and Japan (Sugita et al 1981;Vasconcelos & Lee 1972).…”

“…The mechanism of uptake of coliform bacteria by various bivalves is well established (Cabelli & Heffernan 1970) but the effect of environmental factors and physiological activity of the shellfish on retaining aquatic microorganisms is not known. When oysters were purified in seawater irradiated by ultraviolet light, bacteria of sewage origin were selectively eliminated, leaving a heterotrophic population which may be regarded as part of the natural flora of the gut (Vasconcelos & Lee 1972). Little information is available on the nature of such flora although Colwell & Liston (1960) first suggested the existence of a 'defined commensal flora' in oysters.…”

Bacterial Flora in Pasteurized Oysters after Refrigerated Storage

“…The facultatively anaerobic organisms appearing at five month storage, in descending order, were Bacillus, Staphylococcus, Listeria and Chxtridium. Vasconcelos and Lee (1972) have found Grampositive cocci and Bacillus to be part of the stable flora in Crussostrea g&as subjected to depuration with ultraviolet irradiated seawater. Listeria was also found which had not been detected previously, while Bacillus and Clostridium were still present.…”

Behavior of pathogenic bacteria in the oyster, Crassostrea commercialis, during depuration, re-laying, and storage

“…This would imply mechanisms for selectively retaining some microbial species in the alimentary tract while eliminating others with the feces. This concept is supported by the fact that total bacteriological counts of oysters do not decrease to zero levels when oysters are allowed to feed for extended periods in sterilized water, as is the case during depuration (30,36). As found elsewhere (29,30) and also in the present study, total counts of depurated oysters were generally around 104 cells per g. This ability of oysters to selectively retain some microbial species while eliminating others has profound public health implications when considering the microbiological purification of oysters by either depuration or re-laying and where the elimination of an indicator organism, such as E. coli, is used to judge the elimination of other pathogenic species.…”

“…Bacteriological examinations of oysters have shown the predominant flora to consist of Pseudomonas, Vibrio, Aeromonas, Moraxella, Acinetobacter, Flavobacterium, and Cytophaga species, and it has been suggested that oysters might maintain an indigenous gut flora (6,36). This would imply mechanisms for selectively retaining some microbial species in the alimentary tract while eliminating others with the feces.…”