Automated Rapid Screening of Foods for the Presence of Salmonellae

Abstract

We previously reported an accelerated method to detect salmonellae in foods. It consisted of a preenrichment step (6 h) followed by immunomagnetic separation (IMS) and automated detection, based on positive biochemical characteristics, during overnight incubation. We describe here a simplified procedure with a detection limit of 2 CFU/25 g of food, a procedure that does not require the IMS step. Liquid eggs, milk, ice cream, or poultry were inoculated with Salmonella or non-Salmonella Enterobacteriaceae organisms (2 to 100 CFU/25 g). Following preenrichment for 6 h at 35°C, samples (3 ml) were added to selective liquid media (6 ml). Automated light transmittance was recorded during incubation at 42°C in the BioSys instrument. Salmonella-positive samples were identified by a sharp drop in transmittance (caused by black discoloration of the media) within 18 h of incubation following the formation of hydrogen sulfide and its reaction with iron ions. Detection of 2 to 10 CFU/25 g of the tested foods was completed in 24 h. A total of 24 Salmonella spp., including heat-injured cells, and 37 non-Salmonella Enterobacteriaceae were tested using the procedure.