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We have tested the influence on the polymerase chain reaction (PCR) of a large number of compounds found in food, in media used for selective propagation of food-borne pathogens or in DNA-extraction methods. PCR was found to be sensitive to large volumes of complex food samples containing high amounts of fat and protein, however, an extraction procedure based on treatment with hot NaOH/SDS reduced the effect significantly. Some culture media (Fraser, MLEB, MRB and Rappaport) interfered with the analysis and for most of the media it was possible to assign the inhibitory effect to one or more individual components. Several compounds (detergents, lysozyme, NaOH, alcohols, EDGA, EGTA) used in DNA extraction procedures were found to have some inhibitory effect. The inhibitory effects need to be taken into consideration when designing new tests.

A rapid and simple assay has been developed which allows specific detection of Listeria monocytogenes within 3.5 h in cultures prepared from suspect food samples and propagated 48 h in selective medium. The assay is based on PCR technology, and uses a specific primer set derived from sequences located down-stream of the hlyA gene. The specificity of the primer set was confirmed by testing 115 L. monocytogenes, 14 L. innocua, 5 L. seeligeri and 4 L. ivanovii isolates. The assay was compared to standard microbiological tests and gave identical results for 83 food samples, including 32 positives. These field trials indicate that the assay developed provides an alternative detection system for L. monocytogenes in foods, which can be used by the food industry.