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

Abstract

Oysters (Crassostrea commercialis) harvested from major cultivation areas within the state of New South Wales, Australia, were commonly contaminated with low levels of the food-poisoning organisms Bacillus cereus, Clostridium perfringens, and Vibrio parahaemolyticus. Salmonella was found in oysters on only one occasion. These bacteria were cleansed from oysters during oyster purification by relaying in a non-polluted waterway. Oysters were laboratory contaminated to levels in excess of 1,000 cells per g with either B. cereus, C. perfringens, V. parahaemolyticus, Salmonella typhimurium, or S. senftenberg. These species were cleansed from such oysters during purification in a laboratory depuration unit that used ultraviolet light for sterilizing the depuration water. Escherichia coli was also cleansed from oysters under the same relaying or depuration conditions so that its measurement alone could be used to indicate the cleansing of the above pathogenic species. The levels of these bacteria were also measured during the storage of oysters under conditions that occur during marketing. While B. cereus counts remained relatively stable during storage, the Salmonella spp. gradually decreased in numbers and C. perfringens rapidly died off. V. parahaemolyticus counts increased slightly during the first 4 days of storage, after which decreases occurred.

Relaying to Decrease the Concentration of Oyster-Associated Pathogens

“…In general, oysters were found to reduce S. typhimurium to a low level within < 5 d after relaying, and to a non-detectable level ( < 1 per 200 g) within 5 to 9 d under a wide variety of environmental conditions. Similar results have been obtained by other researchers (21,28). S. montevideo responded with an initial reduc- tion to levels of ^1 cell per gram in 3 to 8 d after relaying, but persisted in oysters at a detectable level (>1 to <200 cells/200 g) for as long as 28 d. Failure of the oysters to eliminate this species is not understood, but may signal that low levels of other species of salmonellae could persist for lengthy periods in oysters, or that failure to detect other salmonellae species is related to the techniques of recovery.…”

“…Relaying waters may contain some indicator bacteria and this study suggested that fecal coliforms may not be useful as end-point indicators for this method of oyster purification. Depuration (7,9) and relaying (3,25,28) are methods…”

“…The filter feeding mechanism of the oyster (10,11,18) is the process by which contamination occurs. Bacteria (13,21,26,27,28), viruses (5,6,8,13,22,30), and other microorganisms as well as foodstuff (23) are ingested.…”

“…The rapidity by which contaminating organisms are excreted depends on the quality of the water being filtered. Salmonellae elimination can occur as rapidly as 3 d (28) if optimal conditions exist or be extended to as long as 49 d (17). Elimination of enteric viruses by shellfish during depuration (12) varies extensively (24 h to 7 d), and depends upon temperature, salinity, and/or the type of depuration system.…”

Changes in the composition and diversity of the bacterial microbiota associated with oysters (Crassostrea corteziensis,Crassostrea gigasandCrassostrea sikamea) during commercial production

“…Next, the shells were washed with 70% ethanol and were depurated for 72 h in filtered, UVsterilised seawater with constant aeration according to the scheduled depuration process (FDA, 1992). The transient microbiota was removed from the oysters using methods previously reported by Son and Fleet (1980) and Lee et al (2008). The external valves were thoroughly cleaned to remove surface contamination, and the oysters were then carefully opened, leaving the animal intact.…”