Specificity of a conductance assay for enumeration of Escherichia coli from broiler carcass rinse samples containing genetically similar species

Experiments were conducted to evaluate the specificity of a rapid method fo r enumeration of Escherichia coti from fresh broiler chicken carcasses. In three separate trials, E. coli, Citrobacter freundii, Salmonella Enteritidi s, and Shigella sonnei were serially diluted and then inoculated into ident ical broiler chicken carcass rinses. Inoculated rinses were mixed with doub le-strength Coliform Medium supplemented with 2% dextrose. This mixture was placed in a Bactometer module in duplicate, and conductance was measured a t 44 degrees C. Results indicated that C. freundii did not grow to an appre ciable degree in the selective medium at 44 degrees C. Salmonella Enteritid is grew similarly to E. coli; however, an initial level of 10(6) Salmonella in the food product would be required for Salmonella to interfere with enu meration 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 line s formed when serial dilutions (log(10) CFU/ml) were compared to E. coli de tection 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. I n general, Salmonella and Shigella are not found at high enough levels on p oultry products to interfere with enumeration of E. coli using this method and, if found at high levels, would be detected and rejected using this pro cedure. Hence, the presence of organisms that are genetically and phylogene tically similar to E. coli would not preclude enumeration of E. coli using conductance under these conditions.