A qPCR-MPN method for rapid quantification of Escherichia coli in bivalve molluscan shellfish

Faecal contamination of bivalve molluscan shellfish (BMS) can lead to infections from enteric pathogens if consumed. Across Europe, the faecal indicator bacteria Escherichia coli, is used to determine contamination of BMS harvesting areas. The reference most probable number (MPN) method for E. coli in BMS takes around 48 h from sample receipt to result. In this study, an alternative method was developed in which the final, E. coli confirmation step in the MPN method (usually carried out on chromogenic TBX agar) was replaced by presence/absence real-time PCR (qPCR). This qPCR-MPN method was directly compared with the reference TBX-MPN method using 194 BMS samples consisting of mussels (Mytilus spp.), Pacific oysters (Crassostrea gigas) and common cockles (Cerastoderma edule). The qPCR-MPN method correlated positively with the TBX-MPN method (Kendall's tau coefficient = 0.812). However, the strength of this correlation varied between BMS species, with mussels having the poorest correlation (0.677) followed by Pacific oysters (0.795) and common cockles (0.890). There were some samples for which the difference between the two methods was higher than might be expected by statistical probability alone. Variations in the way in which the two confirmation methods work may account for much of this variation. This method may serve as an ad hoc, rapid assessment method that is complementary to the official reference method and could be easily implemented in many official control laboratories.