Formalin-fixed cells as an internal standard approach for the detection and quantitative assessment of Shiga toxin-producing Escherichia coli (STEC)

Several Shiga toxin-producing Escherichia coli (STEC) are considered adulterants in raw, non-intact ground meat, and numerous detection assays for these microorganisms have been investigated. The objective of this research was to determine the efficacy of formalin-fixed STEC cells as a potential means to calibrate various detection assays. Eight and ten percent formalin-fixed cells were compared with quantitative PCR (qPCR), and the results suggest that the ten percent formalin fixed cell populations were more stable for quantitative assays. These results were based on the approximation of cells/mL by means of phase contrast microscopy. Fixed cells were spiked into ground beef samples, and the recovery rate for each STEC serotype was determined by qPCR. Additionally, the log cells/mL of spiked samples were determined by qPCR and flow cytometry. The log cells/mL evaluated for each assay were determined to not be statistically significant, which suggests that formalin fixed cells could be used across a wide range of detection platforms (immuno-based versus molecular). Commercial detection kits could potentially incorporate formalin-fixed cells as a more standardized positive control to calibrate detection platforms simultaneously as well as among laboratories.