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. edulis and C. gigas were 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 in M. edulis (1.88Log) and C. gigas (1.27Log) after 24h. Somatic coliphages were bioaccumulated to the highest concentration in both M. edulis (4.84log) and C. gigas (1.73log) after 48h. F-RNA phages were bioaccumulated to the highest concentration in M. edulis (3.51log) after 6h but were below detection limit in C. gigas throughout the exposure period. Escherichia coli, faecal coliforms and intestinal enterococci were bioaccumulated to the highest concentrations in M. edulis (5.05log, 5.06log and 3.98log respectively) after 48h. In C. gigas, E. coli reached a maximum concentration (5.47log) after 6h, faecal coliforms (5.19log) after 12h and intestinal enterococci (3.23log) after 24h.M. edulis bioaccumulated phages to a greater extent than the faecal bacteria, and in both shellfish species, faecal bacteria persisted for longer periods over 48h 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.