Calibration of the impedance method for rapid quantitative estimation of Escherichia coli in live marine bivalve molluscs

Aim: Calibration of impedance measurement was performed vs the Association Francoise de Normalisation (AFNOR) MPN method with a view to rapid enumeration of Escherichia coli in live marine bivalve molluscs. Methods and Results: Linear regression models between log(10) MPN and detection time (DT) were adjusted for several shellfish types, growth media, and impedance instruments (BacTrac and Malthus systems). Escherichia coli concentrations could be estimated from DT using a single regression line for BacTrac 4100 with M1 medium (R-2 = 87.8%) and Malthus with M2 medium (R-2 = 86.7%), and two regression lines for BacTrac 4110 with M2 medium (R-2 = 86.4 and 88.2%). The uncertainty of the predicted bacterial concentration was around +/-0.43 log unit for duplicate sample analysis. The impedance signal was attributable to E. coli in 99% of cases. All cultures containing E. coli produced an impedance signal with BacTrac 4100 and BacTrac 4110, whereas 5.6% did not exhibit a signal with Malthus. Conclusions: Impedance measurement is a possible alternative to the MPN method for rapid quantitative estimation of E. coli in live bivalve shellfish. Significance and Impact of the Study: The impedance method reduces analysis handling time considerably and is much easier to use than the MPN method. Moreover, results can be obtained within 5-10 h, allowing rapid intervention to ensure public health protection in case of shellfish contamination.