Acquisition of Listeria monocytogenes from powdered dairy products using ELISA and DNA gene probe assays prior to, and following, freezing of samples at -10 degrees C and -70 degrees C.

Abstract

Listeria monocytogenes is a human pathogen commonly found in soil, water, vegetation, and foods such as meats, poultry, and dairy products. Over the past two decades, there has been an increase in disease caused by Listeria. Research has shown that Listeria monocytogenes can survive heating, freezing, and other harsh conditions. The injured cells have the capability to repair and multiply under favorable conditions, and thus, regain their ability to cause disease. Therefore, there is a need to identify injured bacterial cells as well as healthy cells. Cultivation based methods developed by the Food and Drug Administration and the United States Department of Agriculture take from 3-28 days to determine if a food product is contaminated with Listeria monocytogenes. An enzyme-linked immunosorbent assay (ELISA) and a deoxyribonucleic acid hybridization assay (DNAH) have been developed to analyze food contamination more rapidly than the standard culture methods. This study compared the ELISA and the DNAH assay by testing dairy products that had been inoculated with Listeria monocytogenes and then exposed to freezing conditions. The ELISA identified all positive samples, whereas the DNAH assay identified all but three positives. Both assays mis-identified only one negative sample each. This study also compared four plating agars used in the determination of Listeria. Trypticase soy agar-yeast extract (TSA-YE) allowed growth on all plates streaked, but gram staining showed that Listeria was not always present because of other competitive flora. Lithium chloride-phenylethanol-moxalactam (LPM) and Modified Oxford medium (MaX) produced the best results. The presence of Listeria was more identifiable on MaX plates.