Purpose: To evaluate QC strains labeled with NanoLuc® Luciferase (Promega) and GFP reporters as positive controls for use in food testing following criteria outlined in the USDA-FSIS protocol for Escherichia coli O157:H7.
Methods: Plasmids bearing either NanoLuc® or GFP reporters were transformed into 8 STEC strains: Big-Six E. coli (ATCC® MP-9™) and E. coli O157:H7 toxigenic (ATCC® 35150™) and non-toxigenic (ATCC®43888™) strains. The stability of the reporters was determined through unselective serial passage. The growth rate, chromogenic properties, and molecular profile of reporter-labeled and native strains were compared.
Results: The signal strength of both reporters was readily detectable by the visual inspection of cells grown in broth culture and on agar plates. Following unselective serial passage, the reporters could be detected in ≥70-100% of colonies for ≥ 2 days. Doubling times between reporter-labeled strains and native strains were tested in triplicate and varied between 0% and 39.4%, in a strain-specific manner. No changes were observed in the chromogenic properties of the reporter-labeled or native strains grown on Rainbow Agar™. Further, the PCR profiles of the stx1, stx2 and eaeA genes were unchanged between the reporter-labeled and native strains.
Significance: This study demonstrates that the NanoLuc® and GFP reporter labeled QC strains can be routinely used as positive controls for STEC detection assays.