Abstract
In order to establish a method for accurately detecting small amounts of Listeria monocytogenes from raw meat, combinations of two kinds of enrichment media, EB medium as defined by the FDA method and Listeria Enrichment Broth (Merck), with four kinds of selective plating media, Oxford Formulation (Oxoid), LPM medium, McBride medium (Oxoid) and Listeria Selective Agar (Merck), were studied. In the recovery test of L. monocytogenes inoculated at less than 10 cells per one gram of minced pork meat, the combination of EB medium for the enrichment medium and Oxford Formulation or LPM medium for the selective medium demonstrated superior detection performances. In the test on 116 samples of commercial minced meat, L. monocytogenes was detected in 44 samples (37.9%) with all the combinations with the highest detection rate of 29.3% (34 positive samples) being obtained by the method of 7-day enrichment culture with EB medium followed by isolation with Oxford Formulation. Isolation with LPM medium was also successful in 33 samples. Of these, however, 6 samples were positive only when isolated on day 2 of enrichment culture, suggesting a need for carrying out isolation culture twice on days 2 and 7 of enrichment culture. Of the selective media, Oxford Formulation was particularly superior in that Listeria colonies could be macroscopically observed by aesculin hydrolysis. The detection rate obtained by the cold-enrichment method was considerably lower than that for 30°C enrichment culture with EB medium.
