Foodborne and Indicator Bacteria in Farmed Molluscan Shellfish before and after Depuration

Galicia's coast (northwestern Spain) is a major producer of bivalve molluscs. Over an 18-month period, the presence of Salmonella, Aeromonas, Plesiomonas shigelloides, Vibrio parahaemolyticus, and Clostridium botulinum was determined by PCR methods in mussels (22 batches) and infaunal bivalves (31 batches of clams and cockles) before and after depuration. All batches were harvested from Galician class B harvesting areas where bivalve molluscs must not exceed 4,600 Escherichia coli per 100 g of flesh and liquor in 90% of the samples. Virulence-associated genes of Salmonella (invA), Aeromonas (aerA, hlyA, alt, ast, and laf), P. shigelloides (hugA), V. parahoemolyticus (tdh and trh), and C. botulinum (BoNT) were not detected. The pR72H chromosomal DNA fragment, which is conservative in V. parahaemolyticus strains, was detected in five (4.7%) samples. A number of 192 suspect isolates did not fit the description of clinical Aeromonas phenospecies, pathogenic Vibrio spp., or P. shigelloides. The effectiveness of commercial depuration in reducing bacterial indicators was also examined. E. coli was reduced to <= 230/100 g of flesh and liquor in 90.9% of mussel lots but in only 70.9% of infaunal bivalve lots. For total coliform elimination, mussels were also more effective. Total Counts significantly (P < 0.005) correlated with numbers of Pseudomonas, Aeromonas, and Vibrio. Our data indicate that Salmonella and pathogenic bacteria indigenous to estuarine environments do not appear to be significant hazards in Galician molluscan shellfish. A reason for concern, however, is that clearance of E. coli to acceptable levels was not always achieved especially in infaunal bivalves.