Rapid and specific detection of Listeria monocytogenes in smoked salmon with BAX®-PCR

Members of the genus Listeria are ubiquitous, and are therefore also common to the food and the environment. Among them, only Listeria monocytogenes has a pathogenic potential, and can cause infectious diseases (listeriosis) in humans. Conventional microbiological testing methods are labour-intensive and time consuming (4–5 days), and often require a number of different culture media for final isolation and confirmatory tests. In order to overcome these limitations, numerous rapid methods have been developed in recent years. DNA-based methods such as the polymerase chain reaction (PCR) have increasingly been used for rapid and sensitive detection of L. monocytogenes. Among the various available PCR assays, we used the BAX^® system (with two different detection procedures: gel detection and automated detection) to screen for L. monocytogenes in samples of vacuum packaged cold smoked salmon. A total of 27 samples were used for this study. The method was compared to the German standard microbiological detection method according to DIN 11290-1 and -2. Detection of Listeria and L. monocytogenes from salmon samples was performed using Palcam enrichment medium, followed by plating on both Palcam agar and ALOA agar. The BAX^® assay gave identical results for 26 food samples compared to the standard method, including 15 positives. Only in one case the BAX^® system gave a false-positive result, probably due to the amplification of DNA from nonviable cells of L. monocytogenes. In naturally contaminated food samples, the BAX^® method gave good results after 24–48 h. Application of this rapid method is simple and time saving.