Bacteria in bivalve shellfish with special reference to the oyster

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Abstract
The bacterial flora of the Pacific oyster Crassostrea gigas, the sea mussel Perna viridis and the arkshell clam Scapharca cornea differed considereably from that of seawater in numbers and generic composition. The numbers of of heterotrophic bacteria in the bivalve shellfish, including the anaerobes and spore-forming bacteria, were greater than that in the surrounding water. Pseudomonas spp. were the dominant organisms, comprising over one third of the 321 strains characterized after isolation from the bivalves and seawater. Other bacteria isolated from the shellfish included Vibrio, acinetobacter and Aeromonas spp., whereas the seawater flora consisted mainly of coliform organisms, coryneform bacteria and Flavobacterium/Cytophaga spp. Bacteria associated with the deposit-feeding clams were higher in density and more distinct in generic composition as compared with those in the suspension-feeding oysters and mussels. Over 90% of the coliform and heterotrophic bacteria in oysters were found in organs associated with the digestive tract. Coliforms were mainly found in the stomach, while heterotrophs were present in stomach and the lower intestine. The stomach flora of oysters are mainly derived from the external environment and, through a process of selection and multiplication that it may be gradually replaced by a more indigenous population which dominates the lower degestive tract.