Evaluation of a Conductance Method for Enumerating Escherichia coli on Chicken, Pork, Fish, Beef, and Milk

Abstract

Experiments were conducted to evaluate a rapid method for enumerating Escherichia coli on food products of animal origin. In study I, rinses from samples of chicken, ground beef, pork, and fish and samples of milk were inoculated with various levels of actively growing E. coli. Conductance assays were monitored at 44°C on each sample using coliform medium supplemented with 2% dextrose. High correlations between E. coli concentrations and E. coli conductance detection times (ECDTs) were found (r = −0.97 to −0.99) for all foods tested in all replicates; however, in most cases, the concordance correlation coefficients (rc) were low, indicating a lack of predictive accuracy. In this study, low accuracy of the conductance method for estimating E. coli counts was attributed to use of concentrations of E. coli that exceed 106 CFU/ml, the detection threshold of the instrument. Slopes of the linear regression lines (E. coli concentration vs. ECDT) for each type of food tested were not significantly different (P < 0.0001), indicating that a single regression equation may be used to estimate E. coli counts for all of the types of food tested in 1 to 7.5 hours using ECDT. In study II, ECDTs for pork, fish, beef, and milk significantly (P < 0.05) decreased in a linear manner as time of temperature abuse increased. Although the ECDT for chicken decreased linearly, no significant differences were observed between 3 and 6 or between 9 and 12 h of abuse. These data demonstrate a strong relationship between increasing populations of E. coli due to temperature abuse and decreasing ECDT. Therefore, results from both studies indicate that this method could be useful for estimating naturally occurring populations of E. coli on foods of animal origin.