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

Abstract

Salmonella spp. and Listeria monocytogenes continue to be major pathogens of 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 adaptation to a simultaneous recovery and detection procedure. Milk, shell eggs, fresh and ready-to-eat (RTE) meats or raw chicken contaminated with a combination of sub-lethally injured salmonellae and listeriae (10–50 cells each) were incubated for 6 h at 35°C in modified universal pre-enrichment broth (MUPB). Volumes (4 ml) were then transferred to vials containing selective liquid media for these pathogens (4 ml), and incubated overnight at 35°C in a BioSys instrument. The presence of the pathogens was identified by a black coloration of the media and a sharp drop in light transmittance caused by hydrogen sulfide production ( Salmonella organisms), or esculin hydrolysis ( Listeria organisms). There was no difference in the detection time of salmonellae when incubated alone or with listeriae, but listeriae grew at a slower rate in the presence of salmonellae, resulting in a delay of ≤1 h in their detection. Overall, the detection of 10–50 salmonellae and 10–50 listeriae in 25 g of the tested foods required 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 ≤30 h for detection and confirmation. Negative samples required no confirmation. The testing system was confirmed in 70 naturally contaminated foods.