Automated simultaneous detection of low levels of listeriae and salmonellae in foods

Salmonella spp. and Listeria monocytogenes continue to be major pathogens o f concern to food processors. However, routine screening of food samples to detect these pathogens is generally labor intensive and costly. Automated optical procedures for the detection of salmonellae and listeriae in foods were developed in our laboratory. In the present study we report their adap tation to a simultaneous recovery and detection procedure. Milk, shell eggs , fresh and ready-to-eat (RTE) meats or raw chicken contaminated with a com bination of sub-lethally injured salmonellae and listeriae (10-50 cells eac h) were incubated for 6 h at 35 degreesC in modified universal pre-enrichme nt broth (MUPB). Volumes (4 mi) were then transferred to vials containing s elective liquid media for these pathogens (4 mi), and incubated overnight a t 35 degreesC in a BioSys instrument. The presence of the pathogens was ide ntified by a black coloration of the media and a sharp drop in light transm ittance caused by hydrogen sulfide production (Salmonella organisms), or es culin hydrolysis (Listeria organisms). There was no difference in the detec tion time of salmonellae when incubated alone or with listeriae, but lister iae grew at a slower rate in the presence of salmonellae, resulting in a de lay of less than or equal to 1 h in their detection. Overall, the detection of 10-50 salmonellae and 10-50 listeriae in 25 g of the tested foods requi red a total of 24 h. Confirmation of the pathogens by PCR-based assay (6 h) was completed the following day directly from positive vials, requiring a total of less than or equal to 30 h for detection and confirmation. Negativ e samples required no confirmation. The testing system was confirmed in 70 naturally contaminated foods. (C) 2001 Elsevier Science B.V. All rights res erved.