Occurrence and detection of viable Listeria in food scrap compost

Listeria species (L. innocua, L. ivanovii, L. seeligeri, and L. grayi) were readily detected in food scraps by Nucleic Acid Hybridization (NAH) probes using a standard Listeria selective medium (UVM-1) at ambient temperature. Various food scrap compost recipes artificially contaminated with Listeria at 10(7) cells per gram wet weight were composted in thermally insulated bench scale reactor vessels. These Listeria were not detected when the compost temperature became elevated. Different isolation methods for the Listeria showed this result to be a false negative occurring apparently because the heat stressed Listeria were unable to survive in the selective medium (UVM-1). Once incubated at 37 degrees C in Universal Listeria medium (ULM), the Listeria were detectable for a short period in compost at temperatures as high as 64 degrees C.