Evaluation of a Reporter-labeled Strain of Salmonella enterica subsp. enterica serovar Typhimurium as a Control in Ground Beef and Media Assays

Introduction: Salmonellosis is the leading cause of death and hospitalization attributed to foodborne illness in the United States and is a significant problem worldwide.  In Salmonella food testing, to effectively discriminate between positive controls and actual food adulterant contamination, strains should contain unique and easily detectable reporters. 

Purpose: To evaluate the use of a Salmonella enterica subsp. enterica serovar Typhimurium strain labeled with NanoLuc^®Luciferase (Promega) as a positive control for use in media and ground beef testing.  

Methods: A plasmid encoding the NanoLuc^® reporter was transformed into Salmonella Typhimurium strain CDC 6516-60 (ATCC^® 14028™).  The stability of the reporter was determined through unselective serial passage and plated on tryptic soy agar.  Following the FDA Bacteriological Analytical Manual for Salmonella (Chapter 5) protocol, the labeled Salmonella Typhimurium was spiked into ground beef samples and grown in lactose broth overnight and used as an inoculum for tetrathionate broth and Rappaport-Vassiliadis broth before being subsequently plated on xylose lysine deoxycholate agar and Hektoen enteric agar.  The process was also performed in the absence of ground beef as a media-only control.

Results: Following unselective daily serial passage, the reporter was visible for ≥ 3 days in 100% of tested colonies grown on tryptic soy agar.  The signal strength of the reporter was readily detectable by visual inspection of cells grown on agar plates from both the media-only and the media with ground beef assays. 

Significance: This study demonstrates that the NanoLuc^®-labeled Salmonella Typhimurium strain can be routinely used as a positive control for media testing and in the detection of microbial pathogens in food.