Simple oPSU broth-BAX® PCR-PicoGreen® system for rapid detection of Listeria monocytogenes in pasteurized milk and hot dogs

Factors that significantly affect BAX® PCR detection of Listeria monocytogenes from optimized Penn State University (oPSU) broth were identified and optimized. Concentration of PCR product was significantly reduced by BAX™ protease and significantly increased by eliminating the lysis step and directly diluting oPSU broth prior to PCR. A simple oPSU broth-BAX® PCR-PicoGreen® (PSU-BAX-PicoGreen) system was developed and compared with current methods for detecting low levels of L. monocytogenes in commercial milk and hot dogs. All 30 milk samples inoculated with 10-20 CFU per ml L. monocytogenes were positive by FDA, BAX® and PSU-BAX-PicoGreen methods and all 42 uninoculated milk samples were negative by all of the above methods. All 30 hot dog samples inoculated with 10-20 CFU/g L. monocytogenes were positive by the USDA and PSU-BAX-PicoGreen methods, however, 2 hot dog samples gave indeterminate results with the standard BAX® method. All 42 uninoculated hot dog samples were negative by USDA, 9 were indeterminate by BAX®and 2 were positive by PSU-BAX-PicoGreen. The PSU-BAX-PicoGreen system may provide a simple and accurate method for rapidly screening pasteurized foods for both injured and noninjured L. monocytogenes.