EVALUATION OF LUCIFERASE REPORTER BACTERIOPHAGE A511-LUXAB FOR DETECTION OF LISTERIA-MONOCYTOGENES IN CONTAMINATED FOODS

A511::luxAB is a recombinant derivative of a broad-host range bacterio phage specific for the genus Listeria, transducing bacterial biolumine scence into infected cells. In this study, we have evaluated its use f or rapid and easy testing of contaminated foods and environmental samp les for the presence of viable Listeria cells, in comparison to the st andard plating procedure. With a short preenrichment step of 20 h, the system was capable of detecting very low initial contamination rates in several foods artificially contaminated with Listeria monocytogenes Scott A cells. In ricotta cheese, chocolate pudding, and cabbage, les s than one cell per g of food could be clearly identified by comparing the light emission of phage-infected samples to that of controls with out lux phage. In foods having a large and complex microbial backgroun d flora, such as minced meat and soft cheese, at least 10 cells per g were necessary to produce a positive bioluminescence signal. Of 348 po tentially contaminated natural food and environmental samples, 55 were found to be Listeria positive by the fur phage method. The standard p lating procedure detected 57 positive samples. Some differences were o bserved with respect to the individual samples, i.e., the lux phage pr ocedure detected more positive samples among the dairy products and en vironmental samples, whereas the plating procedure revealed more conta minated meat and poultry samples. Overall, both methods performed simi larly, i.e., were equally sensitive. However, the minimum time require d for detection of Listeria with the luciferase phage assay was 24 h, which is much shorter than the 4 days needed by the standard plating m ethod. Furthermore, a most probable number technique with three parall els, based on the use of A511::luxAB for differentiation of positive a nd negative tubes, is described. The method enables rapid enumeration of low levels of Listeria cells in several foods tested, against the b ackground of a competing microflora.