Bioaccumulation and persistence of faecal bacterial and viral indicators in Mytilus edulis and Crassostrea gigas

This study investigated the response of two shellfish species − mussels (Mytilus edulis) and oysters (Crassostrea gigas) to microbial contamination in order to gain a better understanding of the bioaccumulation and persistence of microorganisms under controlled conditions.M. edulisandC. gigaswere placed in sterile laboratory-prepared artificial seawater and initial tests were carried out to ensure both faecal indicator bacteria (FIB) and bacteriophages were below detection limits. FIB and phages were isolated, purified and dosed into experimental tanks containing the shellfish species. The GB124 phages were bioaccumulated to the highest concentration inM. edulis(1.88 log10) andC. gigas(1.27 log10) after 24hrs. Somatic coliphages were bioaccumulated to the highest concentration in bothM. edulis(4.84 log10) andC. gigas(1.73 log10) after 48hrs. F-RNA phages were bioaccumulated to the highest concentration inM. edulis(3.51 log10) after 6hrs but were below detection limit inC. gigasthroughout the exposure period.E. coli, faecal coliforms and intestinal enterococci were bioaccumulated to the highest concentrations inM. edulis(5.05 log10, 5.06 log10and 3.98 log10,respectively) after 48hrs. InC. gigas,E. colireached a maximum concentration (5.47 log10) after 6hrs, faecal coliforms (5.19 log10) after 12hrs and intestinal enterococci (3.23 log10) after 24hrs.M. edulisbioaccumulated phages to a greater extent than the faecal bacteria, and in both shellfish species, faecal bacteria persisted for longer periods over 48hrs than the phages. This study highlights significant variation in the levels and rate of accumulation and persistence with respect to both shellfish species and the indicators used to assess risk. The results suggest that phage-based indicators could help elucidate risks to human health associated with pathogenic organisms.