Universal Preenrichment Broth for the Simultaneous Detection of Salmonella and Listeria in Foods

A medium and method for preenriching food products which allow the simultaneous recovery and detection of Salmonella and Listeria are described. To prevent the pH of the medium from rapidly dropping in the presence of extraneous microorganisms found in foods, this medium, universal preenrichment (UP) broth, is highly buffered and low in carbohydrates. The medium allows sublethally injured bacteria to resuscitate and multiply to sufficiently high numbers so that highly selective, secondary enrichment media can be employed to help select the specific bacteria in question from a mixed bacterial background culture. As few as 10 heat-injured Salmonella multiplied to at least 10⁶/ml following a 24 h enrichment in UP, even in mixtures of high levels of known competitive microflora or from naturally occurring microflora found in chicken, hot dogs, or Brie cheese. As few as 10 heat-injured Listeria monocytogenes multiplied to at least 10⁵/ml in these same experiments. From the UP broth, secondary selective preenrichment broths which favor the growth of Salmonella or Listeria can be inoculated, and subsequent protocols for the recovery of either Salmonella or Listeria can then be followed.