Specificity of a Conductance Assay for Enumeration of Escherichia coli from Broiler Carcass Rinse Samples Containing Genetically Similar Species

Abstract

Experiments were conducted to evaluate the specificity of a rapid method for enumeration of Escherichia coli from fresh broiler chicken carcasses. In three separate trials, E. coli, Citrobacter freundii, Salmonella Enteritidis, and Shigella sonnei were serially diluted and then inoculated into identical broiler chicken carcass rinses. Inoculated rinses were mixed with double-strength Coliform Medium supplemented with 2% dextrose. This mixture was placed in a Bactometer module in duplicate, and conductance was measured at 44°C. Results indicated that C. freundii did not grow to an appreciable degree in the selective medium at 44°C. Salmonella Enteritidis grew similarly to E. coli; however, an initial level of 106 Salmonella in the food product would be required for Salmonella to interfere with enumeration of E. coli using this method. S. sonnei grew at a more rapid rate than E. coli; however, there was an interaction between the regression lines formed when serial dilutions (log10 CFU/ml) were compared to E. coli detection times for these two species of bacteria. Therefore, high levels of S. sonnei in a food sample may interfere with the enumeration of E. coli. In general, Salmonella and Shigella are not found at high enough levels on poultry products to interfere with enumeration of E. coli using this method and, if found at high levels, would be detected and rejected using this procedure. Hence, the presence of organisms that are genetically and phylogenetically similar to E. coli would not preclude enumeration of E. coli using conductance under these conditions.