As filter feeders bivalves are exposed to and vulnerable to a large number of pathogenic bacteria. Bivalves in a healthy state are known to harbour a diverse community of microorganisms within their haemolymph thought to confer a health benefit to the host. This study addressed the anti-microbial activity of cultivable haemolymph-associated bacteria within four species of bivalves: oyster Crassostrea gigas; pink clam Tapes rhomboids; blue mussel Mytilus edulis and the scallop Pecten maximus and their potential role as probiotics.
Overall 843 strains of haemolymph-associated bacteria were isolated though bacteria were species and individual dependant. Fixed bivalves, oyster and mussels, had higher haemolymph bacterial concentrations than mobile bivalves with many individual clams and scallops possessing bacterial concentrations below detectable levels. 26 of the 843 strains showed antibacterial activity against at least 1 of the tested pathogens, though antibacterial activity was limited Gram-negative bacteria. None of the isolated strains were sourced from the clam.
As a non-mobile species it has a high bacterial concentration and all strains from this species showed antibacterial potency against at least 5Vibrio spp. which holds huge importance as Vibro pss. are pathogenic to many economically important species including fish, molluscs and crustaceans.
Furthermore the 5 Pseudoalteromonas oyster strains were the only ones tested for cytotoxicity and antibiotic resistance. All these factors together suggest that Pseudoalteromonas isolated from Crassostrea gigas haemolymph is potentially an effective probiotic for aquaculture.